Unit 5 & 6 Final Exam

antigen

A substance that reacts with antibody molecules and antigen receptors on lymphocytes.

State 2 functions of platelets

Although not white blood cells, platelets (thrombocytes) are another formed element in the blood. They promote clotting by sticking together after becoming activated and forming platelet plugs to close up damaged capillaries. They also secrete cytokines and chemokines to promote inflammation

Briefly describe the process involved in the development of antibiotic-associated colitis.

Antibiotic-associated colitis is especially common in older adults. It is thought that C. difficile survives the exposure to the antibiotic by sporulation. After the antibiotic is no longer in the body, the endospores germinate and C. difficile overgrows the intestinal tract and secretes toxin A and toxin B that have a cytotoxic effect on the epithelial cells of the colon. C. difficile has become increasingly resistant to antibiotics in recent years making treatment often difficult. There has been a great deal of success in treating the infection with fecal transplants, still primarily an experimental procedure. Polymerase chain reaction (PCRs) assays, which test for the bacterial gene encoding toxin B, are highly sensitive and specific for the presence of a toxin-producing Clostridium difficile organism. The most successful technique in restricting C. difficile infections has been the restriction of the use of antimicrobial agents.

basophils

Basophils normally make up 0-1% of the WBCs (25-100/mm3 of blood). They are called basophils because their granules stain a dark purplish blue with the basic dye methylene blue, one of the dyes that are used when staining leukocytes. Basophils have a lobed nucleus. a. Basophils release histamine, leukotrienes, and prostaglandins, chemicals that promotes inflammation by causing vasodilation, increasing capillary permeability, and increasing mucous production. Basophils also produce heparin, platelet-activating factor (PAF), and the cytokine IL-4. b. Their life span is probably a few hours to a few days.

State what antigens are composed of chemically.

Chemically, antigens are large molecular weight proteins (including conjugated proteins such as glycoproteins, lipoproteins, and nucleoproteins) and polysaccharides (including lipopolysaccharides). These protein and polysaccharide antigens are found on the surfaces of viruses and cells, including microbial cells (bacteria, fungi, protozoans) and human cells.

Briefly describe the role of autophagy in removing intracellular microbes.

Intracellular microbes, such as viruses and bacteria that invade host cells, can also be engulfed once they enter the cytosol of the cell by a process called autophagy. A membrane-bound compartment called an autophagosome grows around the microbe and the surrounding cytosol and subsequently delivers it to lysosomes for destruction. (This process is also used by eukaryotic cells to engulf and degrade unnecessary or dysfunctional cellular components such as damaged organelles.)

T4-lymphocytes

T4-lymphocytes (CD4+ T-lymphocytes) have CD4 molecules and T-cell receptors (TCRs) on their surface for protein antigen recognition. They function to regulate the adaptive immune responses through cytokine production. Once activated, they differentiate into effector T4-lymphocytes such as Th1 cells, Th2 cells, and Th17 cells

lysozyme

found in in tears, mucous, saliva, plasma, tissue fluid, etc., breaks down peptidoglycan in bacteria causing osmotic lysis. Specifically, it breaks the bond between the N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), the two sugars that make up the backbone of peptidoglycan

Describe the following steps in phagocytosis: a. activation b. chemotaxis c. attachment (both unenhanced and enhanced) d. ingestion e. destruction

1. Activation of the Phagocyte Resting phagocytes are activated by inflammatory mediators such as bacterial products (bacterial proteins, capsules, LPS, peptidoglycan, teichoic acids, etc.), complement proteins, inflammatory cytokines, and prostaglandins. As a result, the circulating phagocytes produce surface glycoprotein receptors that increase their ability to adhere to the inner surface of capillary walls, enabling them to squeeze out of the capillary and be attracted to the site of infection. In addition, they produce endocytic pattern-recognition receptors that recognize and bind to pathogen-associated molecular patterns or PAMPs - components of common microbial molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, and mannose-rich glycans that are not found in human cells - to attach the microbe to the phagocyte for what is called unenhanced attachment. They also exhibit increased metabolic and microbicidal activity by increasing their production of ATPs, lysosomal enzymes, lethal oxidants, etc. 2. Chemotaxis of Phagocytes (for wandering macrophages, neutrophils, and eosinophils) Chemotaxis is the movement of phagocytes toward an increasing concentration of some attractant such as bacterial factors (bacterial proteins, capsules, LPS, peptidoglycan, teichoic acids, etc.), complement proteins (C5a), chemokines (chemotactic cytokines such as interleukin-8 secreted by various cells), fibrin split products, kinins, and phospholipids released by injured host cells. 3. Attachment of the Phagocyte to the Microbe or Cell Attachment of microorganisms is necessary for ingestion. Attachment may be unenhanced or enhanced. a. Unenhanced attachment Unenhanced attachment is the innate recognition of pathogen-associated molecular patterns or PAMPs - components of common molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, mannans, and glucans common in microbial cell walls but not found on human cells - by means of endocytic pattern-recognition receptors, such as scavenger receptors and mannose receptors, on the surface of the phagocytes b. Enhanced attachment Enhanced attachment is the attachment of microbes to phagocytes by way of an antibody molecule called IgG, the complement proteins C3b and C4b produced during the complement pathways, and acute phase proteins such as mannose-binding lectin (MBL) and C-reactive protein (CRP). Molecules such as IgG, C3b, and mannose-binding lectin (MBL) that promote enhanced attachment are called opsonins and the process is also known as opsonization. Enhanced attachment is much more specific and efficient than unenhanced. c. Extracellular trapping with NETs In response to certain pathogen associated molecular patterns such as LPS, and certain cytokines such as IL-8, neutrophils release DNA and antimicrobial granular proteins. These neutrophil extracellular traps (NETs) bind to bacteria, prevent them from spreading, and kill them with antimicrobial proteins 4. Ingestion of the Microbe or Cell by the Phagocyte Following attachment, polymerization and then depolymerization of actin filaments send pseudopods out to engulf the microbe and place it in an endocytic vesicle called a phagosome. During this process, an electron pump brings protons (H+) into the phagosome. This lowers the pH within the phagosome to 3.5 - 4.0 so that when a lysosome fuses with the phagosome, the pH is correct for the acid hydrolases to effectively break down cellular proteins. The acidification also releases defensins, cathelicidin, and bacterial permeability inducing protein (BPI), peptides and enzymes that can kill microbes, from a matrix and enabling their activation. 5. Destruction of the Microbe or Cell Phagocytes contain membranous sacs called lysosomes produced by the Golgi apparatus that contain various digestive enzymes, microbicidal chemicals, and toxic oxygen radicals. The lysosomes travel along microtubules within the phagocyte and fuse with the phagosomes containing the ingested microbes and the microbes are destroyed

State the functions of the following acute phase proteins: a. C-reactive protein b. mannose-binding lectin

1. C-reactive protein (CRP) binds to the phosphorylcholine portion of teichoic acids and lipopolysaccharides of bacterial and fungal cell walls. It also binds to the phosphocholine found on the surface of damaged or dead human cells. It functions as an opsonin, sticking the microorganism to phagocytes, and activates the classical complement pathway by binding C1q, the first component in the pathway. 2. Mannan-binding lectin (MBL) - also known as mannan-binding protein or MBP - binds to mannose-rich glycans (short carbohydrate chains with the sugar mannose or fructose as the terminal sugar). These are common in microbial glycoproteins and glycolipids but rare in those of humans. It functions as an opsonin, sticking the microorganism to phagocytes, and activates the lectin pathway.

endogenous antigen

1. Endogenous antigens are proteins found within the cytosol of human cells. Examples of endogenous antigens include: a. viral proteins produced during viral replication; b. proteins produced by intracellular bacteria such as Rickettsias and Chlamydias during their replication; c. proteins that have escaped into the cytosol from the phagosome of phagocytes such as antigen-presenting cells; d. tumor antigens produced by cancer cells; and e. self-peptides from host cellular proteins

State which body cells display MHC-I surface molecules and which cells normally display MHC-II surface molecules.

1. MHC-I molecules present epitopes to T8-lymphocytes. 2. MHC-II molecules presents epitopes to T4-lymphocytes. The expression of MHC molecules is increased by cytokines produced during both innate immune responses and adaptive immune responses. Cytokines such as interferon-alpha, interferon-beta, interferon-gamma, tumor necrosis factor increase the expression of MHC-I molecules, while interferon-gamma is the main cytokine to increase the expression of MHC-II molecules.

Name 2 endocytic PRRs

1. Mannose receptors Mannose receptors on the surface of phagocytes bind to various microbial carbohydrates such as those rich in mannose or fucose, and to N-acetylglucosamine (NAG). Human glycoproteins and glycolipids typically have terminal N-acetylglucosamine and sialic acid groups. C-type lectins found on the surface of phagocytes are mannose receptors (see Fig. 3). It is now thought that mannose receptors may be quite important in removing potentially harmful mannose-containing glycoproteins such as lysosomal hydrolases that are produced in increased amounts during inflammation. 2. Dectin-1 Dectin-1 recognizes beta-glucans (polymers of glucose) commonly found in fungal cell walls. 3. Scavenger receptors Scavenger receptors found on the surface of phagocytic cells bind to bacterial cell wall components such as LPS, peptidoglycan and teichoic acids (see Fig. 1). There are also scavenger receptors for certain components of other types of microorganisms, as well as for stressed, infected, or injured cells . Scavenger receptors include CD-36, CD-68, and SRB-1. 4. Opsonin receptors Opsonins are soluble molecules produced as a part of the body's immune defenses that bind microbes to phagocytes. One portion of the opsonin binds to a PAMP on the microbial surface and another portion binds to a specific receptor on the phagocytic cell. •Acute phase proteins circulating in the plasma, such as: ◦mannose-binding lectin (also called mannose-binding protein) that binds to various microbial carbohydrates such as those rich in mannose or fucose, and to N-acetylglucosamine (NAG); and ◦C-reactive protein (CRP) that binds to phosphorylcholine portion of teichoic acids and lipopolysaccharides of bacterial and fungal cell walls. It also binds to the phosphocholine found on the surface of damaged or dead human cells. •Complement pathway proteins, such as C3b (see Fig. 4) and C4b recognize a variety of PAMPS. •Surfactant proteins in the alveoli of the lungs, such as SP-A and SP-D are opsonins. •During adaptive immunity, the antibody molecule IgG can function as an opsonin 5. N-formyl Met receptors N-formyl methionine is the first amino acid produced in bacterial proteins since the f-met-tRNA in bacteria has an anticodon complementary to the AUG start codon. This form of the amino acid is not typically seen in mammalian proteins. FPR and FPRL1 are N-formyl receptors on neutrophils and macrophages. Binding of N-formyl Met to its receptor promotes the motility and the chemotaxis of these phagocytes. It also promotes phagocytosis

Compare the oxygen-dependent and oxygen-independent killing systems of neutrophils and macrophages.

1. The oxygen-dependent system: production of reactive oxygen species (ROS) The cytoplasmic membrane of phagocytes contains the enzyme oxidase which converts oxygen into superoxide anion (O2-). This can combine with water by way of the enzyme dismutase to form hydrogen peroxide (H2O2) and hydroxyl (OH) radicals. In the case of neutrophils, but not macrophages, the hydrogen peroxide can then combine with chloride (Cl2-) ions by the action of the enzyme myeloperoxidase (MPO) to form hypochlorous acid (HOCL), and singlet oxygen. In macrophages, nitric oxide (NO) can combine with hydrogen peroxide to form peroxynitrite radicals. (In addition to ROS and NO, macrophages secrete inflammatory cytokines such as TNF-alpha, IL-1, IL-8, and IL-12 to promote an inflammatory response.) These compounds are very microbicidal because they are powerful oxidizing agents which oxidize most of the chemical groups found in proteins, enzymes, carbohydrates, DNA, and lipids. Lipid oxidation can break down cytoplasmic membranes. Collectively, these oxidizing free radicals are called reactive oxygen species (ROS). Oxidase also acts as an electron pump that brings protons (H+) into the phagosome. This lowers the pH within the phagosome so that when lysosomes fuse with the phagosome, the pH is correct for the acid hydrolases, like elastase, to effectively break down cellular proteins. In addition to phagocytes using this oxygen-dependant system to kill microbes intracellularly, neutrophils also routinely release these oxidizing agents, as well as acid hydrolases, for the purpose of killing microbes extracellularly. These agents, however, also wind up killing the neutrophils themselves as well as some surrounding body cells and tissues as mentioned above. 2. The oxygen-independent system Some lysosomes contain defensins, cationic peptides that alter cytoplasmic membranes; lysozyme, an enzyme that breaks down peptidoglycan, lactoferrin, a protein that deprives bacteria of needed iron; cathepsin G, a protease that causes damage to microbial membranes; elastase, a protease that kills many types of bacteria; cathelicidins, proteins that upon cleavage are directly toxic to a variety of microorganisms; bactericidal permeability inducing protein (BPI), proteins used by neutrophils to kill certain bacteria by damaging their membranes; collagenase; and various other digestive enzymes that exhibit antimicrobial activity by breaking down proteins, RNA, phosphate compounds, lipids, and carbohydrates.

Briefly describe the beneficial effects of the following complement pathway products: a. C5a b. C3a c. C3b d. C4b e. C3d f. C5b6789n (MAC)

1. Triggering inflammation C5a is the most potent complement protein triggering inflammation. It reacts with blood vessels causing vasodilation. It also causes mast cells to release vasodilators such as histamine, increasing blood vessel permeability as well as increasing the expression of adhesion molecules on leukocytes and the vascular endothelium so that leukocytes can squeeze out of the blood vessels and enter the tissue (diapedesis). C5a also causes neutrophils to release toxic oxygen radicals for extracellular killing and induces fever. To a lesser extent C3a and C4a also promote inflammation. As we will see later in this unit, inflammation is a process in which blood vessels dilate and become more permeable, thus enabling body defense cells and defense chemicals to leave the blood and enter the tissues. 2. Chemotactically attracting phagocytes to the infection site C5a also functions as a chemoattractant for phagocytes. Phagocytes will move towards increasing concentrations of C5a and subsequently attach, via their CR1 receptors to the C3b molecules attached to the antigen. This will be discussed in greater detail later in this unit under phagocytosis. 3. Promoting the attachment of antigens to phagocytes (enhanced attachment or opsonization) C3b and to a lesser extent, C4b can function as opsonins, that is, they can attach antigens to phagocytes. One portion of the C3b binds to proteins and polysaccharides on microbial surfaces; another portion attaches to CR1 receptors on phagocytes, B-lymphocytes, and dendritic cells for enhanced phagocytosis (see Fig. 8). In actuality, C3b molecule can bind to pretty much any protein or polysaccharide. Human cells, however, produce Factor H that binds to C3b and allows Factor I to inactivate the C3b. On the other hand, substances such as LPS on bacterial cells facilitate the binding of Factor B to C3b and this protects the C3b from inactivation by Factor I. In this way, C3b does not interact with our own cells but is able to interact with microbial cells. C3a and C5a increase the expression of C3b receptors on phagocytes and increase their metabolic activity. 4. Causing lysis of Gram-negative bacteria, human cells displaying foreign epitopes, and viral envelopes C5b6789n, functions as a Membrane Attack Complex (MAC). This helps to destroy gram-negative bacteria as well as human cells displaying foreign antigens (virus-infected cells, tumor cells, etc.) by causing their lysis. It can also damage the envelope of enveloped viruses. 5. Serving as a second signal for activating naive B-lymphocytes during adaptive immunity Some C3b is converted to C3d. C3d binds to CR2 receptors on B-lymphocytes. This serves as a second signal for the activation of B-lymphocytes whose B-cell receptors have just interacted with their corresponding antigen. 6. Removing harmful immune complexes from the body C3b and to a lesser extent, C4b help to remove harmful immune complexes from the body. The C3b and C4b attach the immune complexes to CR1 receptors on erythrocytes. The erythrocytes then deliver the complexes to fixed macrophages within the spleen and liver for destruction. Immune complexes can lead to a harmful Type III hypersensitivity, as will be discussed later in Unit 5 under Hypersensitivities.

Briefly describe two specific examples of how an improper functioning PRR can lead to an increased risk of a specific infection or disease.

2. Most people that die as a result of Legionnaire's disease have been found to have a mutation in the gene coding for TLR-5 that enables the body to recognize the flagella of Legionella pneumophila. 3. B-lymphocytes, the cells responsible for recognizing foreign antigens and producing antibodies against those antigens, normally don't make antibodies against the body's own DNA and RNA. The reason is that any B-lymphocytes that bind the body's own antigens normally undergo apoptosis, a programmed cell suicide. People with the autoimmune disease systemic lupus erythematosis have a mutation in a gene that signals the cell to undergo apoptosis. As a result, these B-cells are able to bind and engulf the body's own DNA and RNA and place them in an endosome or phagolysosome where the the DNA can be recognized by TLR-9 and the RNA by TLR-7. This, in turn, triggers those B-lymphocytes to make antibody molecules against the body's own DNA and RNA. Another gene error enables these B- cells to increase the expression of TLR-7. 4. TLR-4, MyD88, TLR-1/TLR-2 have been implicated in the production of artherosclerosis in mice and some humans. 5. Mutations resulting in loss-of-function in the gene coding for NOD-2 that prevents the NOD-2 from recognizing muramyl dipeptide make a person more susceptible to Crohn's disease, an inflammatory disease of the large intestines. Mutations resulting in over-activation in the gene coding for NOD-2 can lead to an inflammatory disorder called Blau syndrome. 6. People with chronic sinusitis that does not respond well to treatment have decreased activity of TLR-9 and produce reduced levels of human beta-defensin 2, as well as mannan-binding lectin needed to initiate the lectin complement pathway. 7. Pathogenic strains of Staphylococcus aureus producing leukocidin and protein A, including MRSA, cause an increased inflammatory response. Protein A, a protein that blocks opsonization and functions as an adhesin, binds to cytokine receptors for TNF-alpha. It mimics the cytokine and induces a strong inflammatory response. As the inflammatory response attracts neutrophils to the infected area, the leukocidin causes lysis of the neutrophils. As a result, tissue is damaged and the bacteria are not phagocytosed. 8. People with chronic mucocutaneous candidiasis disease have a mutation either in the gene coding for IL-17F or the gene encoding IL-17F receptor. TH17 cells secrete cytokines such as IL-17 that are important for innate immunity against organisms that infect mucous membranes. 9. A polymorphism in the gene for TLR-2 makes individuals less responsive to Treponema pallidum and Borrelia burgdorferi and possibly more susceptible to tuberculosis and staphylococcal infections. 10. Polymorphisms in a gene locus called A20, a gene that helps to control inflammation, are considered as risk alleles for rheumatoid arthritis, systemic lupus erythematosis, psoriasis, type I diabetes, and Chron's disease. 11. The innate immune response to Mycobacterium tuberculosis and the severity of tuberculosis depends on the response of TLR 1/2, TLR 1/6, and 9 to the bacterium. Polymorphisms in Toll-interacting protein (TOLLIP), a negative regulator of TLR signaling, influence the response of the patient to M. tuberculosis.

hapten

A hapten is a small molecule that by itself is not immunogenic but can act as an antigen when it binds to a larger protein molecule. The hapten then acts as an epitope on the protein. For example with penicillin and poison ivy allergies, the penicillin molecules and the oil urushiol from the poison ivy plant function as haptens, binding to tissue proteins to form an antigen and stimulating an allergic immune response.

Name 2 signaling PRRs found on host cell surfaces

A series of signaling pattern-recognition receptors known as toll-like receptors (TLRs) are found on the surface of a variety of defense cells and other cells. These TLRs play a major role in the induction of innate immunity and contribute to the induction of adaptive immunity. Different combinations of TLRs appear in different cell types and may occur in pairs. Different TLRs directly or indirectly bind different microbial molecules. For example: a. TLR-2 - recognizes peptidoglycan, bacterial lipoproteins, lipoteichoic acid (Gram-positive bacteria), and porins (gram-negative bacteria). b. TLR-4 - recognizes lipopolysaccharide (Gram-negative bacteria), fungal mannans, viral envelope proteins, parasitic phospholipids, heat-shock proteins. c. TLR-5 - recognizes bacterial flagellin; d. TLR-1/TLR-2 pairs - binds to bacterial lipopeptides, lipomannans (mycobacteria) lipoteichoic acids (Gram-positive bacteria), cell wall beta glucans (bacteria and fungi), zymosan (fungi) and glycosylphosphatidylinositol (GPI)-anchored proteins (protozoa). e. TLR-2/TL6 pairs - also binds to bacterial lipopeptides, lipomannans (mycobacteria) lipoteichoic acids (Gram-positive bacteria), cell wall beta glucans (bacteria and fungi), zymosan (fungi) and glycosylphosphatidylinositol (GPI)-anchored proteins (protozoa). Another cell surface PRR is CD14. CD14 is found on monocytes, macrophages, and neutrophils and promotes the ability of TLR-4 to respond to LPS. LPS typically binds to LPS-binding protein in the plasma and tissue fluid. The LPS-binding protein promotes the binding of LPS to the CD14 receptors. At that point the LPS-binding protein comes off and the LPS-CD14 bind to TLR-4. Interaction of LPS and CD14 with TLR-4 leads to an elevated synthesis and secretion of inflammatory cytokines such as IL-1, IL-6, IL-8, TNF-alpha, and platelet-activating factor (PAF). These cytokines then bind to cytokine receptors on target cells and initiate inflammation and activate both the complement pathways and the coagulation pathway

State the significance of the following: a*. an elevated white blood cell count b*. a shift to the left (elevated bands)

A. In general, an elevated WBC count (leukocytosis ) is seen in infection, inflammation, leukemia, and parasitic infestations. A decreased WBC count (leukopenia) is generally seen in bone marrow depression, severe infection, viral infections, autoimmune diseases, malignancies, and malnutrition. For example, infections may increase the total leukocyte count two to three times the normal level by dramatically increasing the number of neutrophils. B. when doing a differential WBC count, neutrophils are usually divided into segs (a mature neutrophil having a segmented nucleus) and bands (an immature neutrophil with an incompletely segmented or banded nucleus). During an active infection, people are generally producing large numbers of new neutrophils and therefore will have a higher percentage of the immature band forms. (An increase in band forms is sometimes referred to as a "shift to the left" because on laboratory slips used for differential WBC counts, the heading for bands is to the left of the heading for mature neutrophils or segs.)

Describe what is meant by anatomical barriers to infection.

Anatomical barriers are tough, intact barriers that prevent the entry and colonization of many microbes. Examples include the skin, the mucous membranes, and bony encasements. 1. The skin The skin, consisting of the epidermis and the dermis, is dry, acidic, and has a temperature lower than 37 degrees Celsius (body temperature). These conditions are not favorable to bacterial growth. Resident normal microbiota of the skin also inhibits potentially harmful microbes. In addition, the dead, keratinized cells that make up the surface of the skin are continuously being sloughed off so that microbes that do colonize these cells are constantly being removed. Hair follicles and sweat glands produce lysozyme and toxic lipids that can kill bacteria. Epithelial cells also produce defensins and cathelicidins to kill microbes. Beneath the epidermis of the skin are Langerhans' cells - immature dendritic cells - that phagocytose and kill microbes, carry them to nearby lymph nodes, and present antigens of these microbes to T-lymphocytes to begin adaptive immune responses against them. Finally, intraepithelial T-lymphocytes and B-1 lymphocytes are associated with the epidermis and the mucosal epithelium. These cells recognize microbes common to the epidermis and mucous membranes and start immediate adaptive immune responses against these commonly encountered microbes. 2. The mucous membranes Mucous membranes line body cavities that open to the exterior, such as the respiratory tract, the gastrointestinal tract, and the genitourinary tract. Mucous membranes are composed of an epithelial layer that secretes mucus, and a connective tissue layer. The mucus is a physical barrier that traps microbes. Mucus also contains lysozyme to degrade bacterial peptidoglycan, an antibody called secretory IgA that prevents microbes from attaching to mucosal cells and traps them in the mucous, lactoferrin to bind iron and keep it from from being used by microbes, and lactoperoxidase to generate toxic superoxide radicals that kill microbes. Resident normal microbiota of the mucosa also inhibits potentially harmful microbes. In addition, the mucous membrane, like the skin, is constantly sloughing cells to remove microbes that have attached to the mucous membranes. Beneath the mucosal membrane is mucosa-associated lymphoid tissue (MALT) that contains Langerhans' cells - immature dendritic cells - that phagocytose and kill microbes, carry them to nearby lymph nodes, and present antigens of these microbes to T-lymphocytes to begin adaptive immune responses against them. Intraepithelial T-lymphocytes and B-1 lymphocytes are associated with the epidermis and the mucosal epithelium. These cells recognize microbes common to the epidermis and mucous membranes and start immediate adaptive immune responses against these commonly encountered microbes. 3. Bony encasements Bony encasements, such as the skull and the thoracic cage, protect vital organs from injury and entry of microbes

autoantigen

Autoantigens are any of an organism's own antigens (self-antigens) that stimulate an autoimmune reaction, that is humoral immunity or cell-mediated against self.

B-lymphocytes

B-lymphocytes (B-cells) mediate humoral immunity, the production of antibody molecules against a specific antigen,and have B-cell receptors (BCR) on their surface for antigen recognition. Generally 10-20% of the lymphocytes are B-lymphocytes. Once activated, most B-lymphocytes differentiate into antibody-secreting plasma cells.

Describe how bacterial antagonism by normal microbiota acts as a non-specific body defense mechanism and name 2 opportunistic microbes that may cause superinfection upon destruction of the normal microbiota.

Bacterial antagonism is the process by which the body's normal microbiota inhibit the growth or colonization of opportunistic pathogens and pathogens. Approximately 100 trillion bacteria and other microorganisms reside in or on the human body. The normal body microbiota keeps potentially harmful opportunistic pathogens in check and also inhibits the colonization of pathogens by: 1. Producing metabolic products (fatty acids, bacteriocins, etc.) that inhibit the growth of many pathogens; 2. Adhering to target host cells so as to cover them and preventing pathogens from colonizing; 3. Depleting nutrients essential for the growth of pathogens; and 4. Non-specifically stimulating the immune system. Destruction of normal bacterial microbiota by the use of broad spectrum antibiotics may result in superinfections or overgrowth by antibiotic-resistant opportunistic microbiota. For example, the yeast Candida, that causes infections such as vaginitis and thrush, and the bacterium Clostridium difficile, that causes potentially severe antibiotic-associated colitis, are opportunistic microorganisms normally held in check by the normal microbiota. In the case of Candida infections, the Candida resists the antibacterial antibiotics because being a yeast, it is eukaryotic, not prokaryotic like the bacteria. Once the bacteria are eliminated by the antibiotics, the Candida has no competition and can overgrow the area. Clostridium difficile is an opportunistic Gram-positive, endospore-producing bacillus transmitted by the fecal-oral route that causes severe antibiotic-associated colitis. C. difficile is a common healthcare-associated infection (HAIs) and is the most frequent cause of health-care-associated diarrhea. C. difficile infection often recurs and can progress to sepsis and death. C. difficile infection often recurs and can progress to sepsis and death. CDC has estimated that there are about 500,000 C. difficile infections (CDI) in health-care associated patients each year and is linked to 15,000 American deaths each year.

List 3 groups of noninfectious materials that may act as an antigen.

Certain non-infectious materials may also act as antigens if they are recognized as "nonself" by the body. These include: a. allergens, including dust, pollen, hair, foods, dander, bee venom, drugs, and other agents causing allergic reactions; b. foreign tissues and cells from transplants and transfusions; and c. the body's own cells that the body fails to recognize as "normal self," such as cancer cells, infected cells, and autoantigens involved in autoimmune diseases.

chemokines

Chemokines are a group of cytokines that enable the migration of leukocytes from the blood to the tissues at the site of inflammation. They increase the affinity of integrins on leukocytes for ligands on the vascular wall during diapedesis, regulate the polymerization and depolymerization of actin in leukocytes for movement and migration, and function as chemoattractants for leukocytes. In addition, they trigger some WBCs to release their killing agents for extracellular killing and induce some WBCs to ingest the remains of damaged tissue. Certain chemokines promote angiogenesis. Chemokines also regulate the movement of B-lymphocytes, T-lymphocytes, and dendritic cells through the lymph nodes and the spleen. When produced in excess amounts, chemokines can lead to damage of healthy tissue as seen in such disorders as rheumatoid arthritis, pneumonia, asthma, adult respiratory distress syndrome (ARDS), and septic shock. Examples of chemokines include IL-8, MIP-1a, MIP-1b, MCP-1, MCP-2, MCP-3, GRO-a, GRO-b, GRO-g, RANTES, and eotaxin. Chemokines are produced by many cells including leukocytes, endothelial cells, epithelial cells, and fibroblasts.

Briefly describe the problems that arise from chronic inflammation.

Chronic inflammation, however, can result in considerable tissue damage and scarring. With prolonged increased capillary permeability, neutrophils continually leave the blood and accumulate in the tissue at the infected or injured site. As they discharge their lysosomal contents and reactive oxygen species or ROS, surrounding tissue is destroyed and eventually replaced with scar tissue. Anti-inflammatory agents such as antihistamines or corticosteroids may have to be given to relieve symptoms or reduce tissue damage. For example, as learned in Unit 3, during severe systemic infections with large numbers of microorganisms present, high levels of pathogen-associated molecular patterns (PAMPs) are released resulting in excessive cytokine production by macrophages and this can harm the body. In addition, neutrophils start releasing their proteases and reactive oxygen species that kill not only the bacteria, but the surrounding tissue as well. Harmful effects include high fever, hypotension, tissue destruction, wasting, acute respiratory distress syndrome or ARDS, disseminated intravascular coagulation or DIC, damage to the vascular endothelium, hypovolemia, and reduced perfusion of blood through tissues and organs resulting to shock, multiple system organ failure (MOSF), and often death. This excessive inflammatory response is referred to as Systemic Inflammatory Response Syndrome or SIRS or the Shock Cascade. Chronic inflammation also contributes to heart disease, Alzheimer's disease, diabetes, and cancer.

cytokines

Cytokines are low molecular weight, soluble proteins that are produced in response to an antigen and function as chemical messengers for regulating the innate and adaptive immune systems. They are produced by virtually all cells involved in innate and adaptive immunity, but especially by T- helper (TH) lymphocytes. The activation of cytokine-producing cells triggers them to synthesize and secrete their cytokines. The cytokines, in turn, are then able to bind to specific cytokine receptors on other cells of the immune system and influence their activity in some manner. Cytokines are pleiotropic, redundant, and multifunctional. Some cytokines are antagonistic in that one cytokine stimulates a particular defense function while another cytokine inhibits that function. Other cytokines are synergistic wherein two different cytokines have a greater effect in combination than either of the two would by themselves. There are three functional categories of cytokines: 1. cytokines that regulate innate immune responses, 2. cytokines that regulate adaptive Immune responses, and 3. cytokines that stimulate hematopoiesis. Cytokines that regulate innate immunity are produced primarily by mononuclear phagocytes such as macrophages and dendritic cells, although they can also be produced by T-lymphocytes, NK cells, endothelial cells, and mucosal epithelial cells. They are produced primarily in response to pathogen-associated molecular patterns (PAMPs) such as LPS, peptidoglycan monomers, teichoic acids, unmethylated cytosine-guanine dinucleotide or CpG sequences in bacterial and viral genomes, and double-stranded viral RNA. Cytokines produced in response to PRRs on cell surfaces, such as the inflammatory cytokines TNF-alpha, IL-1, IL-6, and IL-8, mainly act on leukocytes and the endothelial cells that form blood vessels in order to promote and control early inflammatory responses. Cytokines produced in response to PRRs that recognize viral nucleic acids, such as type I interferons, primarily block viral replication within infected host cells

State how long it takes for early induced innate immunity to become activated and what it involves.

Early induced innate immunity begins 4 - 96 hours after exposure to an infectious agent and involves the recruitment of defense cells as a result of pathogen-associated molecular patterns or PAMPS binding to pattern-recognition receptors or PRRs. These recruited defense cells include: •phagocytic cells: leukocytes such as neutrophils, eosinophils, and monocytes; tissue phagocytic cells in the tissue such as macrophages; •cells that release inflammatory mediators: inflammatory cells in the tissue such as macrophages and mast cells; leukocytes such as basophils and eosinophils; and •natural killer cells (NK cells). Unlike adaptive immunity, innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells.

endocytic pattern recognition receptors

Endocytic pattern-recognition receptors, also called phagocytic pattern-recognition receptors, are found on the surface of phagocytes and promote the attachment of microorganisms to phagocytes leading to their subsequent engulfment and destruction.

eosinophils

Eosinophils normally comprise 1-4% of the WBCs (50-400/mm3 of blood). They are called eosinophils because their granules stain red with the acidic dye eosin, one of the mixture of dyes used when staining leukocytes. The nucleus of an eosinophil typically appears lobbed. a. Their granules contain destructive enzymes for killing infectious organisms. These enzymes include acid phosphatase, peroxidases, major basic protein, RNase, DNases, lipase, and plasminogen. b. They are capable of phagocytosis but primarily they release their contents into the surrounding environment to kill microbes extracellularly. c. The substances they release defend primarily against fungi, protozoa, and parasitic worms (helminths), pathogens that are too big to be consumed by phagocytosis. d. They secrete leukotrienes, prostaglandins, chemicals that promotes inflammation by causing vasodilation and increasing capillary permeability. They also secrete various cytokines such as IL-1, IL-2, IL-4, IL-5, IL-6, IL-8, IL-13, and TNF alpha. e. Their life span is 8-12 days.

Describe the 4 processes that make up the inflammatory mechanism.

Essentially, four processes make up the inflammatory mechanism: a. Smooth muscles around larger blood vessels contract to slow the flow of blood through the capillary beds at the infected or injured site. This gives more opportunity for leukocytes to adhere to the walls of the capillary and squeeze out into the surrounding tissue. b. The endothelial cells that make up the wall of the smaller blood vessels contract. This increases the space between the endothelial cells resulting in increased capillary permeability. Since these blood vessels get larger in diameter as a result of this, the process is called vasodilation c. Molecules called selectins are produced on the membrane of the leukocyte and are able to reversibly bind to corresponding selectin glycoprotein receptors on the inner wall of the venule. This reversible binding enables the leukocyte to roll along the inner wall of the venule. Adhesion molecules are activated on the surface of the endothelial cells on the inner wall of the capillaries. Corresponding molecules on the surface of leukocytes called integrins attach to these adhesion molecules allowing the leukocytes to flatten and squeeze through the spaces between the endothelial cells. This process is called diapedesis or extravasation. d. Activation of the coagulation pathway causes fibrin clots to physically trap the infectious microbes and prevent their entry into the bloodstream. This also triggers blood clotting within the surrounding small blood vessels to both stop bleeding and further prevent the microorganisms from entering the bloodstream.

Define DAMPs and give two examples.

Examples of DAMPs associated with stressed, injured, infected, or transformed host cells and not found on normal cells include: a. heat-shock proteins; b. altered membrane phospholipids; and c. molecules normally located inside phagosomes and lysosomes that enter the cytosol only when these membrane-bound compartments are damaged as a result of infection, including antibodies bound to microbes from opsonization. d. molecules normally found within cells, such as ATP, DNA, and RNA, that spill out of damaged cells.

Name at least 5 PAMPS associated with bacteria.

Examples of microbial-associated PAMPs include: a. lipopolysaccharide (LPS) from the outer membrane of the Gram-negative cell wall b. bacterial lipoproteins and lipopeptides c. porins in the outer membrane of the Gram-negative cell wall d. peptidoglycan found abundantly in the Gram-positive cell wall and to a lesser degree in the gram-negative cell wall e. lipoteichoic acids found in the Gram-positive cell wall f. lipoarabinomannan and mycolic acids found in acid-fast cell walls g. mannose-rich glycans (short carbohydrate chains with the sugar mannose or fructose as the terminal sugar). These are common in microbial glycoproteins and glycolipids but rare in those of humans h. flagellin found in bacterial flagella; i. bacterial and viral nucleic acid. Bacterial and viral genomes contain a high frequency of unmethylated cytosine-guanine dinucleotide or CpG sequences (a cytosine lacking a methyl or CH3 group and located adjacent to a guanine). Mammalian DNA has a low frequency of CpG sequences and most are methylated which may mask recognition by pattern-recognition receptors. Also, human DNA and RNA does not normally enter cellular endosomes where the pattern-recognition receptors for microbial DNA and RNA are located;

exogenous antigen

Exogenous antigens are antigens that enter from outside the body, such as bacteria, fungi, protozoa, and free viruses. These exogenous antigens enter macrophages, dendritic cells, and B-lymphocytes through phagocytosis or pinocytosis.

Briefly describe the healing stage of inflammation.

Finally, within 1 to 3 days, macrophages release the cytokines interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-α). These cytokines stimulate NK cells and T-lymphocytes to produce the cytokine interferon-gamma (IF-γ). The (IF-γ) then binds to receptors on macrophages causing them to produce fibroblast growth factor and angiogenic factors for tissue remodeling. With the proliferation of endothelial cells and fibroblasts, endothelial cells form a fine network of new capillaries into the injured area to supply blood, oxygen, and nutrients to the inflamed tissue. The fibroblasts deposit the protein collagen in the injured area and form a bridge of connective scar tissue to close the open, exposed area. This is called fibrosis or scarring, and represents the final healing stage.

State what happens when either phagocytes are overwhelmed with microbes or they adhere to cells to large to be phagocytosed.

If the infection site contains very large numbers of microorganisms and high levels of inflammatory cytokines and chemokines are being produced in response to PAMPs , the phagocyte will empty the contents of its lysosomes by a process called degranulation in order to kill the microorganisms or cell extracellularly. The phagocyte will also empty the contents of its lysosomes for extracellular killing if the cell to which the phagocyte adheres is too large to be engulfed

Compare immediate innate immunity with early induced innate immunity.

Immediate innate immunity begins 0 - 4 hours after exposure to an infectious agent and involves the action of soluble preformed antimicrobial molecules that circulate in the blood, our found in extracellular tissue fluids, and are secreted by epithelial cells. These include: •antimicrobial enzymes and peptides; •complement system proteins; and •anatomical barriers to infection, mechanical removal of microbes, and bacterial antagonism by normal body microbiota. Early induced innate immunity begins 4 - 96 hours after exposure to an infectious agent and involves the recruitment of defense cells as a result of pathogen-associated molecular patterns or PAMPS binding to pattern-recognition receptors or PRRs. These recruited defense cells include: •phagocytic cells: leukocytes such as neutrophils, eosinophils, and monocytes; tissue phagocytic cells in the tissue such as macrophages; •cells that release inflammatory mediators: inflammatory cells in the tissue such as macrophages and mast cells; leukocytes such as basophils and eosinophils; and •natural killer cells (NK cells).

State how long it takes for immediate innate immunity to become activated and what it involves.

Immediate innate immunity begins 0-4 hours after exposure to an infectious agent and involves the action of soluble preformed antimicrobial molecules that circulate in the blood, our found in extracellular tissue fluids, and are secreted by epithelial cells. These include: •antimicrobial enzymes and peptides; •complement system proteins; and These preformed antimicrobial molecules are designed to immediately begin to remove infectious agents as soon as they enter the body. In addition to preformed antimicrobial molecules, the following also play a role in immediate innate immunity: •anatomical barriers to infection •mechanical removal of microbes •bacterial antagonism by the body's normal microbiota

In terms of infectious diseases, list 2 categories of microbial materials that may act as an antigen.

In terms of infectious diseases, the following may act as antigens: a. microbial structures, such as bacterial and fungal cell walls, protozoan cell membranes, bacterial and fungal capsules, microbial flagella, bacterial pili, viral capsids, viral envelope-associated glycoproteins, etc.; and b. microbial toxins

Compare adaptive (acquired) immunity with innate immunity.

Innate immunity is an antigen-nonspecific defense mechanisms that a host uses immediately or within several hours after exposure to almost any microbe. This is the immunity one is born with and is the initial response by the body to eliminate microbes and prevent infection. Innate immunity can be divided into immediate innate immunity and early induced innate immunity. Unlike adaptive immunity, innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells. These unique microbial molecules are called pathogen-associated molecular patterns or PAMPS and include LPS from the gram-negative cell wall, peptidoglycan and lipotechoic acids from the gram-positive cell wall, the sugar mannose (a terminal sugar common in microbial glycolipids and glycoproteins but rare in those of humans), bacterial and viral unmethylated CpG DNA, bacterial flagellin, the amino acid N-formylmethionine found in bacterial proteins, double-stranded and single-stranded RNA from viruses, and glucans from fungal cell walls. In addition, unique molecules displayed on stressed, injured, infected, or transformed human cells also act as PAMPS. (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)

interferons

Interferons modulate the activity of virtually every component of the immune system. Type I interferons include 13 subtypes of interferon-alpha, interferon-beta, interferon omega, interferon-kappa, and interferon tau. (There is only one type II interferon, interferon-gamma, which is involved in the inflammatory response.) The most powerful stimulus for type I interferons is the binding of viral DNA or RNA to toll-like receptors TLR-3, TLR-7, and TLR-9 in endosomal membranes. 1. TLR-3 - binds double-stranded viral RNA; 2. TLR-7 - binds single-stranded viral RNA, such as in HIV, rich in guanine/uracil nucleotide pairs; 3. TLR-9 - binds unmethylated cytosine-guanine dinucleotide sequences (CpG DNA) found in bacterial and viral genomes but uncommom or masked in human DNA and RNA.

Describe at least 4 ways the body deprives microorganisms of iron.

Iron is needed as a cofactor for certain enzymes in both bacteria and humans. Both bacteria and human cells produce iron chelators that trap free iron from their environment and transport it into the cell. During infection, the body makes considerable metabolic adjustment in order to make iron unavailable to microorganisms. Much of this is due to production of a defense chemical called leukocyte-endogenous mediator (LEM). As a result of infection, there is: 1. Decreased intestinal absorption of iron from the diet; 2. A decrease of iron in the plasma and an increase in iron in storage as ferritin; 3. An increased synthesis of the human iron-binding proteins (iron chelators) such as lactoferrin, transferrin, ferritin, and hemin that trap iron for use by human cells while making it unavailable to most microbes; 4. Coupled with the febrile response, decreased ability of bacteria to synthesize their own iron chelators called siderophores; 5. Prior stationing of lactoferrin at common sites of microbial invasion such as in the mucous of mucous membranes, and the entry of transferrin into the tissue during inflammation. This lack of iron, which is needed as a cofactor for certain enzyme reactions, can inhibit the growth of many bacteria.

Name the cells in the tissue whose primary function is to present antigen to effector T-lymphocytes.

Macrophages primarily capture and present protein antigens to effector T-lymphocytes. (Effector lymphocytes are lymphocytes that have encountered an antigen, proliferated, and matured into a form capable of actively carrying out immune defenses.) Macrophages engulf the microorganism and degrade it with their lysosomes. Peptides from microbial proteins are then bound to a groove of unique molecules called MHC-II molecules produced by macrophages, dendritic cells, and B-lymphocytes. The peptide epitopes bound to the MHC-II molecules are then put on the surface of the macrophage where they can be recognized by complementary shaped T-cell receptors (TCR) and CD4 molecules on an effector T4-lymphocyte. This interaction leads to the activation of that macrophage.

Briefly describe the major difference between the effect of the cytokines produced in response to PAMPs that bind to cell surface signaling PRRs and endosomal PRRs

Many of the TLRs of cell surface signaling PRRs, especially those that bind to bacterial and fungal cell wall components, stimulate the transcription and translation of inflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and interleukin-12 (IL-12), as well as chemokines such as interleukin-8 (IL-8), MCP-1, and RANTES. These cytokines trigger innate immune defenses such as inflammation, fever, and phagocytosis in order to provide an immediate response against the invading microorganism (see Fig. 7). Because cytokines such as IL-I, TNF-alpha, and IL-12 that trigger an inflammatory response, they are often referred to as inflammatory cytokines. Chemokines are a group of cytokines that enable the migration of leukocytes from the blood to the tissues at the site of inflammation. To counter inflammation, anti-inflammatory cytokines such as IL-1 receptor antagonist, IL-4, and IL-10 are produced.

State the primary function of mast cells in body defense.

Mast cells are typically the immunological first responders to infection and carry out many of the same inflammatory-mediating functions as basophils. There are two types of mast cells in the body: mast cells found in the connective tissue and mast cells found throughout the mucous membranes. The granules of mast cells contain such mediators as histamine, eosinophil chemotactic factor, neutrophil chemotactic factor, platelet activating factor, and cytokines such as IL-3, IL-4, IL-5, IL-6, and TNF-alpha. They also possess pathways for synthesizing leukotrienes and prostaglandins, chemicals that promote inflammation by causing vasodilation, increasing capillary permeability, and increasing mucous production. Mast cells have pattern-recognition receptors or PRRs on their surface that interact with pathogen-associated molecular patterns or PAMPs of microbes. After the PAMPs bind to their respective PRRs, they release the contents of their granules. These chemical mediators promote inflammation and attract neutrophils to the infected site.

List 4 ways in which the body can physically remove microorganisms or their products.

Mechanical removal is the process of physically flushing microbes from the body. Methods include: 1. Mucus and cilia Mucus traps microorganisms and prevents them from reaching and colonizing the mucosal epithelium. Mucus also contains lysozyme to degrade bacterial peptidoglycan, an antibody called secretory IgA that prevents microbes from attaching to mucosal cells and traps them in the mucus, lactoferrin to bind iron and keep it from from being used by microbes, and lactoperoxidase to generate toxic superoxide radicals that kill microbes. Cilia on the surface of the epithelial cells propel mucus and trapped microbes upwards towards the throat where it is swallowed and the microbes are killed in the stomach. This is sometimes called the tracheal toilet. 2. The cough and sneeze reflex Coughing and sneezing removes mucus and trapped microbes. 3. Vomiting and diarrhea These processes remove pathogens and toxins in the gastrointestinal tract. 4.The physical flushing action of body fluids Fluids such as urine, tears, saliva, perspiration, and blood from injured blood vessels also flush microbes from the body.

pattern-recognition receptors (PRRs)

Molecules on or in host cells that are able to recognize or bind to pathogen-associated molecular patterns in order to induce innate immunity

pathogen-associated molecular patterns (PAMPs)

Molecules unique to microorganisms that are not associated with human cells. They include LPS, peptidoglycan, lipoteichoic acids, mangoes, flagellin, pilin, bacterial DNA, and viral double-stranded RNA

State what type of cell monocytes differentiate into when they enter tissue.

Monocytes differentiate into macrophages and dendritic cells when they leave the blood and enter the tissue. Macrophages and dendritic cells are very important in phagocytosis and serve as antigen-presenting cells in the adaptive immune responses (see below). They produce a variety of cytokines that play numerous roles in body defense

monocytes

Monocytes normally make up 2-8% of the WBCs (100-500/mm3 of blood). They have a compact nucleus and have no visible cytoplasmic granules a. Monocytes are important phagocytes. b. Monocytes differentiate into macrophages and dendritic cells when they leave the blood and enter the tissue. c. They are long-lived (life span of months) and can multiply.

Name the cells in the tissue whose primary function is to present antigen to naive T-lymphocytes.

Most dendritic cells are derived from monocytes and are referred to as myeloid dendritic cells. They are located throughout the epithelium of the skin, the respiratory tract, and the gastrointestinal tract, as well as lymphoid tissues and organ parenchyma. In these locations, in their immature form, they are attached by long cytoplasmic processes. Upon capturing antigens through pinocytosis and phagocytosis and becoming activated by inflammatory cytokines, the dendritic cells detach from their initial site, enter lymph vessels, and are carried to regional lymph nodes. By the time they enter the lymph nodes, they have matured and are now able to present antigen to the ever changing populations of naive T-lymphocytes located in the cortex of the lymph nodes.

Briefly describe the process of diapedesis, indicating the role of P-selectins, integrins, and adhesion molecules.

Most leukocyte diapedesis (extravasation) occurs in post-capillary venules because hemodynamic shear forces are lower in these venules. This makes it easier for leukocytes to attach to the inner wall of the vessel and squeeze out between the endothelial cells. 1) During the very early stages of inflammation, stimuli such as injury or infection trigger the release of a variety of mediators of inflammation such as leukotrienes, prostaglandins, and histamine. The binding of these mediators to their receptors on endothelial cells leads to vasodilation, contraction of endothelial cells, and increased blood vessel permeability. In addition, the basement membrane surrounding the capillaries becoming rearranged so as to promote the migration of leukocytes and the movement of plasma macromolecules from the capillaries into the surrounding tissue. 2) The binding of histamine to histamine receptors on endothelial cells triggers an upregulation of P-selectin molecules and platelet-activating factor (PAF) on the endothelial cells that line the venules. 3). The P-selectins then are able to reversibly bind to corresponding P-selectin glycoprotein ligands (PSGL-1) on leukocytes. This reversible binding enables the leukocyte to now roll along the inner wall of the venule. 4) The binding of PAF to its corresponding receptor PAF-R on the leukocyte upregulates the surface expression of leukocyte function-associated molecule-1 (LFA-1) on the surface of the leukocyte. 5) The LFA-1 molecules on the rolling leukocytes can now bind firmly to intercellular adhesion molecule-1 (ICAM-1) found on the surface of the endothelial cells forming the inner wall of the blood vessel. 6) The leukocytes flatten out, squeeze between the constricted endothelial cells, and move across the basement membrane as they are are attracted towards chemotactic agents such as the complement protein C5a and leukotriene B4 generated by cells at the site of infection or injury.

NK cells

NK cells (natural killer cells) are lymphocytes that lack B-cell receptors and T-cell receptors. They function to kill infected cells and tumor cells. NK cells are able to kill cells to which antibody molecules have attached through a process called antibody-dependent cellular cytotoxicity (ADCC). They also kill human cells lacking MHC-I molecules on their surface

Describe how NK cells are able to recognize and kill infected cells and cancer cells lacking MHC-I molecules.

NK cells are important in innate immunity because they are able to recognize infected cells, cancer cells, and stressed cells and kill them. In addition, they produce a variety of cytokines, including proinflammatory cytokines, chemokines, colony-stimulating factors, and other cytokines that function as regulators of body defenses. For example, through cytokine production NK cells also suppress and/or activate macrophages , suppress and/or activate the antigen-presenting capabilities of dendritic cells, and suppress and/or activate T-lymphocyte responses. NK cells use a dual receptor system in determining whether to kill or not kill human cells. When cells are either under stress, are turning into tumors, or are infected, various stress-induced molecules such as MHC class I polypeptide-related sequence A (MICA) and MHC class I polypeptide-related sequence B (MICB) are produced and are put on the surface of that cell. The first receptor, called the killer-activating receptor, can bind to these stress-induced molecules, and this sends a positive signal that enables the NK cell to kill the cell to which it has bound unless the second receptor cancels that signal. This second receptor, called the killer-inhibitory receptor, recognizes MHC-I molecules that are usually present on all nucleated human cells. MHC-I molecules, produced by all nucleated cells in the body, possess a deep groove that can bind peptides from proteins found within the cytosol of human cells, transport them to the surface of that cell, and display the MHC-!/peptide complex to receptors on cytotoxic T-lymphocytes or CTLs. If the MHC-I molecules have peptides from the body's own proteins bound to them, CTLs do not recognize those cells as foreign and the cell is not killed. If, on the other hand, the MHC-I molecules have peptides from viral, bacterial, or mutant proteins bound to them, CTLs recognize that cell as foreign and kill that cell. If MHC-I molecules/self peptide complexes are expressed on the cell, the killer-inhibitory receptors on the NK cell recognize this MHC-I/peptide complex and sends a negative signal that overrides the original kill signal and prevents the NK cell from killing the cell to which it has bound.

neutrophils

Neutrophils are the most abundant of the leukocytes, normally accounting for 54-75% of the WBCs. An adult typically has 3,000-7,500 neutrophils/mm3 of blood but the number may increase two- to three-fold during active infections. They are called neutrophils because their granules stain poorly - they have a neutral color - with the mixture of dyes used in staining leukocytes. The nucleus of a neutrophil has multiple lobes. Functions of neutrophils: a. Neutrophils are important phagocytes. b. Their granules contain various agents for killing microbes. Primary azurophil granules contain acid hydrolase, myeloperoxidase, defensins, cathepsin G, cationic proteins, and bactericidal permeability increasing protein (BPI). Secondary specific granules contain such defense chemicals as lysozyme, lactoferrin, collagenase, and elastase. These agents kill microbes intracellularly during phagocytosis but are also often released extracellularly where they kill not only microbes but also surrounding cells and tissue, as will be discussed later under phagocytosis. c. They release the enzyme kallikrein that catalyzes the generation of bradykinins. Bradykinins promote inflammation by causing vasodilation, increasing vascular permeability, and increasing mucous production. They are also chemotactic for leukocytes and stimulate pain. d. They produce enzymes that catalyze the synthesis of prostaglandins from arachidonic acid in cell membranes. Certain prostaglandins promote inflammation by causing vasodilation and increasing capillary permeability. They also cause constriction of smooth muscles, enhance pain, and induce fever. e. They are short-lived, having a life span of a few hours to a few days, and do not multiply. They circulate in the blood for around 6 hours and if the are not recruited, they undergo apoptosis. In tissue, they function for several hours and die. However, the bone marrow makes about 80,000,000 new neutrophils per minute to replace these.

Describe the overall function of iNKT cells in terms how they promote both innate and adaptive immunity and may also help to regulate the immune responses.

Once activated, the iNKT cells rapidly produce large quantities of cytokines, including interferon-gamma (IFN-γ), interleukin-4 (IL-4), interleukin-2 (IL-2), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-α), interleukin-13 (IL-13), and chemokines. Through the rapid productions of such cytokines, iNKT cells are able to promote and suppress different innate and adaptive immune responses. For example, large amounts of IFN-γ are produced by activated iNKT cells. IFN-γ activates NK cells and macrophages as a part of innate immunity. It has been proposed that if the iNKT cell is repeatedly stimulated by the body's own glycolipids in the absence of microbes that this might stimulate the iNKTcell /dendritic cell interaction to produce tolerizing signals that inhibit the TH1 cell response and possibly stimulate the production of regulatory T-lymphocytes (Treg cells). In this way it might suppress autoimmune responses and prevent tissue damage. There is also growing evidence that early childhood exposure to microbes is associated with protection against allergic diseases, asthma, and inflammatory diseases such as ulcerative colitis. It has been found that germ-free mice have large accumulations of mucosal iNKT cells in the lungs and intestines and increased morbidity from allergic asthma and inflammatory bowel disease. However, colonization of neonatal germ-free mice with normal microbiota resulted in mucosal iNKT cell tolerance to these diseases. It has been proposed that microbes the human body has been traditionally exposed to from early childhood throughout most of human history might play a role in developing normal iNKT cell numbers and iNKT cell responses.

Name 2 signaling PRRs found on the host cell cytoplasm

Pattern-recognition receptors or PRRs found in the cytoplasm include: a. NODs (nucleotide-binding oligomerization domain) NOD proteins, including NOD-1 and NOD-2, are cytostolic proteins that allow intracellular recognition of peptidoglycan components. 1. NOD-1 recognizes peptidoglycan containing the muramyl dipeptide NAG-NAM-gamma-D-glutamyl-meso diaminopimelic acid, part of the peptidoglycan monomer in common gram-negative bacteria and just a few gram-positive bacteria. 2. NOD-2 recognizes peptidoglycan containing the muramyl dipeptide NAG-NAM-L-alanyl-isoglutamine found in practically all bacteria (see Fig. 2). As macrophages phagocytose either whole bacteria or peptidoglycan fragments released during bacterial growth, the peptidoglycan is broken down into muramyl dipeptides. Binding of the muramyl dipeptides to NOD-1 or NOD-2 leads to the activation of genes coding for inflammatory cytokines such as IL-1, TNF-alpha, IL-8, and IL-12 in a manner similar to the cell surface TLRs. Activation of NOD-2 also induces the production of antimicrobial peptides such as defensins as well as microbicidal reactive oxygen species (ROS). b. CARD-containing proteins CARD (caspase activating and recruitment domain)-containing proteins, such as RIG-1 (retinoic acid-inducible gene-1) and MDA-5 (melanoma differentiation-associated gene-5), are cytoplasmic sensors of viral RNA molecules that trigger the synthesis of type-1 interferons, antiviral cytokines that block viral replication within infected host cells in a manner similar to the endosomal TLRs. RIG-1 recognizes 5'-PPPs on viral RNAs. The 5'-PPPs on host cell RNAs are either capped or removed and are not recognized by RIG-1. Rig-1 and MDA-5 can also, through another regulatory pathway, stimulate the production of inflammatory cytokines. c. Danger recognition receptors or DRRs Danger recognition receptors or DRRs found in the cytoplasm recognize danger-associated molecular patterns (DAMPS) in the cytosol such as altered membrane phospholipids, and materials released from damaged phagosomes and damaged lysosomes, including antibodies bound to microbes from opsonization. DAMPs are also produced as a result of tissue injury during cancer, heart attack, and stroke. Detection of DAMPs by DRRs in the cytosol also triggers the activation of inflammasomes, release of inflammatory cytokines, and pyroptosis.

Briefly describe the role of the following as they relate to phagocytosis: a. inflammation b. lymph nodules c. lymph nodes d. spleen

Phagocytosis is the primary method used by the body to remove free microorganisms in the blood and tissue fluids. Phagocytic cells include neutrophils, eosinophils, monocytes, macrophages, dendritic cells, and B-lymphocytes. The body's phagocytic cells are able to encounter these microorganisms in a variety of ways: a. Infection or tissue injury stimulates mast cells, basophils, and other cells to release vasodilators to initiate the inflammatory response. Vasodilation results in increased capillary permeability, enabling phagocytic white blood cells such as neutrophils, monocytes, and eosinophils - as well as other leukocytes - to enter the tissue around the injured site. The leukocytes are then chemotactically attracted to the area of infection. In other words, inflammation allows phagocytes to enter the tissue and go to the site of infection. Neutrophils are the first to appear and are later replaced by macrophage. b. Lymph nodules are unencapsulated masses of lymphoid tissue containing fixed macrophages and ever changing populations of B-lymphocytes and T-lymphocytes. They are located in the respiratory tract, the liver, and the gastrointestinal tract and are collectively referred to as mucosa-associated lymphoid tissue or MALT. Examples include the adenoids and tonsils in the respiratory tract and the Peyer's patches on the small intestines. Organisms entering these systems can be phagocytosed by fixed macrophages and dendritic cells and presented to B-lymphocytes and T-lymphocytes to initiate adaptive immune responses. c. Tissue fluid picks up microbes and then enters the lymph vessels as lymph. Lymph vessels carry the lymph to regional lymph nodes. Lymph nodes contain many reticular fibers that support fixed macrophages and dendritic cells as well as ever changing populations of circulating B-lymphocytes and T-lymphocytes. Microbes picked up by the lymph vessels are filtered out and phagocytosed in the lymph nodes by these fixed macrophages and dendritic cells and presented to the circulating B-lymphocytes and T-lymphocytes to initiate adaptive immune responses. The lymph eventually enters the circulatory system at the heart to maintain the fluid volume of the circulation. d. In addition, Langerhans' cells - immature dendritic cells - are located throughout the epithelium of the skin, the respiratory tract, and the gastrointestinal tract where in their immature form they are attached by long cytoplasmic processes. Upon capturing antigens through pinocytosis and phagocytosis and becoming activated by proinflammatory cytokines, the dendritic cells detach from the epithelium, enter lymph vessels, and are carried to regional lymph nodes. By the time they enter the lymph nodes, they have matured and are now able to present antigen to the ever changing populations of naive T-lymphocytes located in the cortex of the lymph nodes. e. The spleen contains many reticular fibers that support fixed macrophages and dendritic cells, as well as ever changing populations of circulating B-lymphocytes and T-lymphocytes. Blood carries microorganisms to the spleen where they are filtered out and phagocytosed by the fixed macrophages and dendritic cells and presented to the circulating B-lymphocytes and T-lymphocytes to initiate adaptive immune responses.

State what is meant by the phrase "Cytokines are pleiotropic, redundant, and multifunctional."

Pleiotropic means that a particular cytokine can act on a number of different types of cells rather than a single cell type. Redundant refers to the ability of a number of different cytokines to carry out the same function. Multifunctional means the same cytokine is able to regulate a number of different functions.

Name 2 signaling PRRs found in the endosomes of phagocytic cells

Signaling PRRs found in the membranes of the endosomes (phagolysosomes) used to degrade pathogens (see Fig. 2): a. TLR-3 - binds double-stranded viral RNA; b. TLR-7 - binds single-stranded viral RNA, such as in HIV, rich in guanine/uracil nucleotide pairs; c. TLR-8 - binds single-stranded viral RNA; d. TLR-9 - binds unmethylated cytosine-guanine dinucleotide sequences (CpG DNA) found in bacterial and viral genomes but uncommom or masked in human DNA and RNA.

Describe specifically how type I interferons are able to block viral replication within an infected host cell.

Signaling pattern recognition receptors located in the cytoplasm of cells such as RIG-1 and MDA-5 also signal synthesis and secretion of type-I interferons. Type I interferons, produced abundantly by plasmacytoid dendritic cells, by virtually any virus-infected cell, and by other defense cells provide an early innate immune response against viruses. Interferons induce uninfected cells to produce an enzyme capable of degrading viral mRNA, as well as one that blocks translation in eukaryotic cells. These enzymes remain inactive until the uninfected cell becomes infected with a virus. At this point, the enzymes are activated and begin to degrade viral mRNA and block translation in the host cell. This not only blocks viral protein synthesis, it also eventually kills the infected cell (see Slideshow Fig. 2A and Fig. 2B). In addition, type I interferons also cause infected cells to produce enzymes that interfere with transcription of viral RNA or DNA. They also promote body defenses by enhancing the activities of CTLs, macrophages, dendritic cells, NK cells, and antibody-producing cells, as well as induce chemokine production to attract leukocytes to the area.

T8-lymphocytes

T8-lymphocytes (CD8+ T-lymphocytes) have CD8 molecules and T-cell receptors (TCRs) on their surface for protein antigen recognition. Once activated, they differentiate into cytotoxic T-lymphocytes (CTLs).

epitope

The actual portions or fragments of an antigen that react with receptors on B-lymphocytes and T-lymphocytes, as well as with free antibody molecules. Usually equivalent to 5-15 amino acids or 3-4 sugar residues

Briefly describe the mechanism behind the acute phase response.

The acute phase response is an innate body defense seen during acute illnesses and involves the increased production of certain blood proteins termed acute phase proteins. Activated macrophages and other leukocytes release inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), and interleukin-6 (IL-6) when their pattern-recognition receptors (PRRs) bind pathogen associated molecular patterns or PAMPs - molecular components associated with microorganisms but not found as a part of eukaryotic cells. These include bacterial molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, mannans, flagellin, pilin, and bacterial DNA. There are also pattern-recognition molecules for viral double-stranded RNA (dsRNA) and fungal cell walls components such as lipoteichoic acids, glycolipids, mannans, and zymosan. The acute phase response is an innate body defense seen during acute illnesses and involves the increased production of certain blood proteins termed acute phase proteins. Activated macrophages and other leukocytes release inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), and interleukin-6 (IL-6) when their pattern-recognition receptors (PRRs) bind pathogen associated molecular patterns or PAMPs - molecular components associated with microorganisms but not found as a part of eukaryotic cells. These include bacterial molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, mannans, flagellin, pilin, and bacterial DNA. There are also pattern-recognition molecules for viral double-stranded RNA (dsRNA) and fungal cell walls components such as lipoteichoic acids, glycolipids, mannans, and zymosan.

Briefly describe how the alternative complement pathway is activated

The alternative complement pathway is mediated by C3b, produced either by the classical or lectin pathways or from C3 hydrolysis by water. (Water can hydrolize C3 and form C3i, a molecule that functions in a manner similar to C3b.) 1. Activation of the alternative complement pathway begins when C3b (or C3i) binds to the cell wall and other surface components of microbes. C3b can also bind to IgG antibodies. Alternative pathway protein Factor B then combines with the cell-bound C3b to form C3bB. Factor D then splits the bound Factor B into Bb and Ba, forming C3bBb. A serum protein called properdin then binds to the Bb to form C3bBbP that functions as a C3 convertase (see Fig. 13) capable of enzymatically splitting hundreds of molecules of C3 into C3a and C3b. The alternative complement pathway is now activated.

Briefly describe how the classical complement pathway is activated.

The classical complement pathway is initiated by activation of C1. C1 is primarily activated by interacting with the Fc portion of the antibody molecules IgG or IgM after they have bound to their specific antigen. C1 is also able to directly bind to the surfaces of some pathogens as well as with the C-reactive protein (CRP) that is produced during the acute phase response of innate immunity.

State what each of the following determine: CBC and leukocyte differential count.

The complete blood count (CBC) is a laboratory test which, among other things, determines the total number of both leukocytes and erythrocytes per ml of blood. The differential white blood cell count (leukocyte differential count) determines the number of each type of leukocyte calculated as a percentage of the total number of leukocytes. This information can be useful diagnostically because different diseases or disorders can cause an increase or a decrease in the various types of WBCs.

Briefly describe the various beneficial effects of inflammation that are associated with plasma leakage and with diapedesis.

The inflammatory response is an attempt by the body to restore and maintain homeostasis after injury and is an integral part of body defense. Most of the body defense elements are located in the blood and inflammation is the means by which body defense cells and defense chemicals leave the blood and enter the tissue around the injured or infected site. Inflammation is essentially beneficial, however, excess or prolonged inflammation can cause harm. As a result of this increased capillary permeability: a. Plasma flows out of the blood into the tissue. Beneficial molecules in the plasma include: 1. Clotting factors. Tissue damage activates the coagulation cascade causing fibrin clots to form to localize the infection, stop the bleeding, and chemotactically attract phagocytes. 2. Antibodies. These help remove or block the action of microbes through a variety of methods that will be explained in Unit 6. 3. Proteins of the complement pathways. These, in turn: 1) stimulate more inflammation (C5a, C3a, and C4a), 2) stick microorganisms to phagocytes (C3b and C4b), 3) chemotactically attract phagocytes ( C5a), and 4) lyse membrane-bound cells displaying foreign antigens (membrane attack complex or MAC). 4. Nutrients. These feed the cells of the inflamed tissue. 5. Lysozyme, cathelicidins, phospholipase A2, and human defensins. Lysozyme degrades peptidoglycan. Cathelicidins are cleaved into two peptides that are directly toxic to microbes and can neutralize LPS from the gram-negative bacterial cell wall. Phospholipase A2 hydrolizes the phospholipids in the bacterial cytoplasmic membrane. Human defensins put pores in the cytoplasmic membranes of many bacteria. Defensins also activate cells involved in the inflammatory response. 6. Transferrin. Transferrin deprives microbes of needed iron. b. Leukocytes enter the tissue through a process called diapedesis or extravasation, discussed above under early inflammation and late inflammation. Benefits of diapedesis include: 1. Increased phagocytosis. Neutrophils, monocytes that differentiate into macrophages when they enter the tissue, and eosinophils are phagocytic leukocytes that enter the tissue. 2. More vasodilation. Basophils, eosinophils, neutrophils, and platelets enter the tissue and release or stimulate the production of vasoactive agents that promote inflammation. 3. Cytotoxic T-lymphocytes (CTLs), effector T4-lymphocytes, and NK cells enter the tissue to kill cells such as infected cells and cancer cells that are displaying foreign antigens on their surface

Briefly describe how the lectin pathway is activated

The lectin pathway is activated by the interaction of microbial carbohydrates (lectins) with mannose-binding lectin (MBL) or ficolins found in the plasma and tissue fluids. The lectin pathway is mediated by two groups of proteins found in the plasma of the blood and in tissue fluids: 1. Mannose-binding lectin (MBL) - also known as mannose-binding protein or MBP. MBL is a soluble pattern-recognition receptor that binds to various microbial carbohydrates such as those rich in mannose or fucose, and to N-acetylglucosamine (NAG). These glycans are common in microbial glycoproteins and glycolipids but rare in those of humans. MBL is synthesized by the liver and released into the bloodstream as part of the acute phase response that will be discussed later in this unit. The MBL is equivalent to C1q in the classical complement pathway. Ficolins are similar in their structure to MBL and bind to microbial carbohydrates such as N-acetylglucosamine (NAG), lipoteichoic acids, and lipopolysaccharide (LPS). Ficolin is also equivalent to C1q in the classical complement pathway. 2. Both mannose-binding lectin (MBL) and ficolin form complexes with MBL-associated serine proteases called MASP1 and MASP2, which are equivalent to C1r and C1s of the classical pathway.

State the primary function of dendritic cells in body defense.

The primary function of dendritic cells is to capture and present protein antigens to naive T-lymphocytes . (Naive lymphocytes are those that have not yet encountered an antigen.) Dendritic cells engulf microorganisms and other materials and degrade them with their lysosomes. Peptides from microbial proteins are then bound to a groove of unique molecules called MHC-II molecules produced by macrophages, dendritic cells, and B-lymphocytes. The peptide epitopes bound to the MHC-II molecules are then put on the surface of the dendritic cell (see Fig. 1) where they can be recognized by complementary shaped T-cell receptors (TCR) and CD4 molecules on naive T4-lymphocyte. In addition, dendritic cells can bind peptide epitopes to MHC-I molecules and present them to naive T8-lymphocytes. The MHC-I molecules with bound peptide on the dendritic cell are recognized by complementary shaped T-cell receptors (TCR) and CD8 molecules on naive T8-lymphocyte. These interactions enable the T4-lymphocytes or T8-lymphocytes to become activated, proliferate, and differentiate into effector cells. Myeloid dendritic cells also use pattern-recognition receptors called toll-like receptors (TLRs) to recognize pathogen-associated molecular patterns or PAMPs. The interaction of the PAMP with its TLR stimulates the production of co-stimulatory molecules that are also required for T-lymphocyte activation. Dendritic cells produce many of the same inflammatory cytokines as macrophages, such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), interleukin-6 (IL-6), and interleukin-8 (IL-8). They also can produce interleukin-12 (IL-12), a cytokine that can activate natural killer T-lymphocytes (NKT cells). Another type of dendritic cell, the plasmacytoid dendritic cell, uses its TLRs to recognize viral PAMPs. This interaction results in the production and secretion of antiviral type-I interferons

Describe how an overactive TLR-4 receptor can increase the risk of SIRS in a person if Gram-negative bacteria enter the bloodstream.

There are a number of harmful effects that are known to occur as a result of either an overactive or an underactive innate immune response. This occurs as a result of people possessing different polymorphisms in the various genes participating in PRR signaling. •People born with underactive PRRs or deficient PRR immune signaling pathways are at increased risk of infection by specific pathogens due to a decrease innate immune response. •People born with overactive PRRs or deficient PRR immune signaling pathways are at increased risk of inflammatory damage by lower numbers of specific pathogens. Examples include: 1. People with an underactive form of TLR-4, the toll-like receptor for bacterial LPS, have been found to be five times as likely to contract a severe bacterial infection over a five year period than those with normal TLR-4. People with overactive TLR-4 receptors may be more prone to developing SIRS from gram-negative bacteria.

Describe the mechanism behind fever induction and indicate its possible benefits.

These cytokines stimulate the anterior hypothalamus of the brain, the part of the brain that regulates body temperature, to produce prostaglandin E2, which leads to an increase bodily heat production and increased vasoconstriction. This, in turn, decreases the loss of heat from the skin and increases body temperature. Up to a certain point, fever is beneficial: 1. Fever increases the environmental temperature above the optimum growth temperature for many microorganisms. If the microorganisms are growing more slowly, the body's defenses have a better chance of removing them all. 2. Fever leads to the production of heat shock proteins that are recognized by some intraepithelial T-lymphocytes called delta gamma T-cells, resulting in the production of inflammation-promoting cytokines. 3. Fever elevates the temperature of the body increasing the rate of enzyme reactions, and speeding up metabolism within the body. An elevation in the rate of metabolism can increase the production and activity of phagocytes, speed up the multiplication of lymphocytes, increase the rate of antibody and cytokine production, increase the rate at which leukocytes are released from the bone marrow into the bloodstream, and speed up tissue repair.

Describe what causes most of the tissue destruction seen during microbial infections.

These released lysosomal contents, however, also kill surrounding host cells and tissue. Most tissue destruction associated with infections is a result of this process.

Define hyperpyrexia.

Too high of a body temperature, however, may cause damage by denaturing the body's enzymes. Hyperpyrexia is a fever with an extreme elevation of body temperature greater than or equal to 41.5 °C (106.7 °F). Body temperature this elevated often indicates a serious underlying condition and may lead to potentially hazardous side effects. As a result, hyperpyrexia is considered as a medical emergency.

State two factors that can result in a nucleated human cell not producing MHC-I molecules.

Viruses, stress, and malignant transformation, however, can often interfere with the ability of the infected cell or tumor cell to express MHC-I molecules. Without the signal from the killer-inhibitory receptor, the kill signal from the killer-activating signal is not overridden and the NK cell kills the cell to which it has bound

State 3 different functions of macrophages in body defense.

When monocytes leave the blood and enter the tissue, they become activated and differentiate into macrophages. Those that have recently left the blood during inflammation and move to the site of infection through positive chemotaxis are sometimes referred to as wandering macrophages. In addition, the body has macrophages already stationed throughout all tissues and organs of the body. These are sometimes referred to as fixed macrophages. Many fixed macrophages are part of the mononuclear phagocytic (reticuloendothelial) system. They, along with B-lymphocytes and T-lymphocytes, are found supported by reticular fibers in lymph nodules, lymph nodes, and the spleen where they filter out and phagocytose foreign matter such as microbes. Similar cells are also found in the liver (Kupffer cells), the kidneys (mesangial cells), the brain (microglia), the bones (osteoclasts), the lungs (alveolar macrophages), and the gastrointestinal tract (peritoneal macrophages). Macrophages actually have a number of very important functions in body defense including: 1. Killing of microbes, infected cells, and tumor cells by phagocytosis. Macrophages that have engulfed microorganisms become activated by a subset of T-helper lymphocytes called Th1 cells (see Fig. 6). Activated macrophages develop a ruffled cytoplasmic membrane and produce increased numbers of lysosomes. 2. Processing antigens so they can be recognized by effector T-lymphocytes during the adaptive immune responses. Macrophages, as well as the dendritic cells mentioned below, process antigens through phagocytosis and present them to T-lymphocytes. Because of this function, they are often referred to as antigen-presenting cells or APCs . 3. Secreting lipid mediators of inflammation such as leukotrienes, prostaglandins, and platelet-activating factor (PAF). 4. Secreting proteins called cytokines that play a variety of roles in non-specific body defense. Macrophage-produced cytokines promote inflammation and induce fever, increase phagocytosis and energy output, promote sleep, activate resting T-lymphocytes, attract and activate neutrophils, and stimulate the replication of endothelial cells to form capillaries and fibroblasts to form connective scar tissue. Four important cytokines that macrophages produce are tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), interleukin-6 (IL-6), and interleukin-8 (IL-8). There is growing evidence that monocytes and macrophages can be "trained" by an earlier infection to do better in future infections, that is, develop memory. It is thought that microbial pathogen-associated molecular patterns (PAMPs) binding to pattern-recognition (PRRs) on monocytes and macrophages triggers the cell's epigenome to reprogram or train that cell to react better against new infections. Macrophages show great functional diversity.

Compare B-cell receptors and T-cell receptors in terms of how they recognize epitopes.

a. B-cell receptors The antigen receptors on the cytoplasmic membrane of B-lymphocytes are called B-cell receptors and are actually antibody molecules made by that cell and anchored to the outer surface of its cytoplasmic membrane. As will be seen in a later section, antibodies are "Y"-shaped macromolecules composed of four glycoprotein chains connected to one another by disulfide (S-S) bonds and noncovalent bonds (see Fig. 4). Additional S-S bonds fold the individual glycoprotein chains into a number of distinct globular domains. The two tips of the "Y" are referred to as the Fab portions of the antibody (see Fig. 4 and Fig. 5). The first 110 amino acids or first domain of both the heavy and light chain of the Fab region of the antibody provide specificity for binding an epitope on an antigen. Because they recognize molecular shapes that occur as a result of the 3-dimensional folding of an antigen, B-cell receptors can bind directly to epitopes on peptide, protein, polysaccharide, nucleic acid, and lipid antigens. The bottom part of the "Y", the C terminal region of each glycoprotein chain, is called the Fc portion. The Fc portion has a constant amino acid sequence that defines the class and subclass of each antibody. The terminal portion of the Fc region of the B-cell receptor is the part that becomes anchored to the cytoplasmic membrane of B-lymphocyte b. T-cell receptors The receptors on the membrane of T-lymphocytes are called T-cell receptors or TCRs. They are analogous to the B-cell receptor, but are composed of just two glycoprotein chains, each having a variable domain and a constant domain. Unlike B-cell receptors that can directly bind to epitopes on antigens, the T-cell receptor or TCR of most T4-lymphocytes and T8-lymphocytes can only recognize peptide epitopes from protein antigens presented by the body's own cells by way of special molecules called MHC molecules as seen in Fig. 6. The terminal portion of the variable domains provides specificity for binding peptides of protein antigens after the protein has been unfolded, broken into peptides, and bound to a MHC molecule, while the teminus of the constant region becomes anchored to the cytoplasmic membrane of the T-lymphocyte. The TCR of CD4-CD8- T-lymphocytes and non-MHC restricted CD4+ and CD8+ lymphocytes can recognize epitopes of lipid or glycolipid antigens after they have been attached to CD1 molecules on antigen-presenting cells or in some cases, epitopes directly on antigens.

Name the two cytokines that are most important in stimulating acute inflammation.

a. Tumor necrosis factor-alpha (TNF-α) TNF-α is the principle cytokine that mediates acute inflammation. In excessive amounts it also is the principal cause of systemic complications such as the shock cascade. Functions include acting on endothelial cells to stimulate inflammation and the coagulation pathway; stimulating endothelial cells to produce selectins and ligands for leukocyte integrins during diapedesis; stimulating endothelial cells and macrophages to produce chemokines that contribute to diapedesis, chemotaxis, and the recruitment of leukocytes; stimulating macrophages to secrete interleukin-1 (IL-1) for redundancy; activating neutrophils and promoting extracellular killing by neutrophils; stimulating the liver to produce acute phase proteins, and acting on muscles and fat to stimulate catabolism for energy conversion. TNF-α stimulates the endothelial cells that form capillaries to express proteins that activate blood clot formation within the capillaries. This occludes local blood flow to help prevent microbes from entering the bloodstream. In addition, TNF is cytotoxic for some tumor cells; interacts with the hypothalamus to induce fever and sleep; stimulates the synthesis of collagen and collagenase for scar tissue formation; and activates macrophages. TNF is produced by monocytes,macrophages, dendritic cells, TH1 cells, and other cells. b. Interleukin-1 (IL-1) IL-1 function similarly to TNF-α in that it mediates acute inflammatory responses. It also works synergistically with TNF to enhance inflammation. Functions of IL-1 include promoting inflammation; activating the coagulation pathway, stimulating the liver to produce acute phase proteins, catabolism of fat for energy conversion, inducing fever and sleep; stimulates the synthesis of collagen and collagenase for scar tissue formation; stimulates the synthesis of adhesion factors on endothelial cells and leukocytes for diapedesis; and activates macrophages. IL-1 is produced primarily by monocytes, macrophages, dendritic cells, endothelial cells, and some epithelial cell.

immunogen

an antigen that is recognized by the body as nonself and stimulates an adaptive immune response.

phospholipase A2

an enzyme that penetrates the bacterial cell wall and hydrolizes the phospholipids in the bacterial cytoplasmic membrane

lactotransferrin and transferrin

found in body secretions, plasma, and tissue fluid, trap iron for use by human cells while preventing its use by microorganisms

State how iNKT cells recognize glycolipids in order to become activated

iNKT cells are a subset of lymphocytes that bridge the gap between innate and adaptive immunity. They have T-cell receptors (TCRs) on their surface for glycolipid antigen recognition. They also have natural killer (NK) cell receptors. Through the cytokines they produce once activated, iNKT cells are essential in both innate and adaptive immune protection against pathogens and tumors. They also play a regulatory role in the development of autoimmune diseases, asthma, and transplantation tolerance. It has been shown that iNKT cell deficiency or disfunction can lead to the development of autoimmune diseases, human asthma, and cancers. Pathogens may not directly activate iNKT cells. The TCR of iNKT cells recognize exogenous glycolipid antigens, as well as endogenous self glycolipid antigens presented by MHC-I-like CD1d molecules on antigen presenting dendritic cells. iNKT cells can also be activated by the cytokine interleukin-12 (IL-12) produced by dendritic cells that have themselves become activated by pathogen-associated molecular patterns (PAMPs) of microbes binding to the pattern-recognition receptors (PRRs) of the dendritic cell.

State what is meant by pathogen-associated molecular patterns (PAMPs), and the role PAMPs play in inducing innate immunity.

innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells. These unique microbial molecules are called pathogen-associated molecular patterns or PAMPS and include LPS from the Gram-negative cell wall, peptidoglycan and lipotechoic acids from the Gram-positive cell wall, the sugar mannose (a terminal sugar common in microbial glycolipids and glycoproteins but rare in those of humans), bacterial and viral unmethylated CpG DNA, bacterial flagellin, the amino acid N-formylmethionine found in bacterial proteins, double-stranded and single-stranded RNA from viruses, and glucans from fungal cell walls.

Briefly describe how intraepithelial T-lymphocytes and B-1 cells play a role in innate immunity.

intraepithelial T-lymphocytes and B-1 lymphocytes are associated with the epidermis and the mucosal epithelium. These cells recognize microbes common to the epidermis and mucous membranes and start immediate adaptive immune responses against these commonly encountered microbes.

Name at least 2 PAMPS associated with viruses.

k. double-stranded viral RNA unique to many viruses in some stage of their replication; l. single-stranded viral RNA from many` viruses having an RNA genome;

pattern recognition receptors (PRRs)

molecules on or in host cells that are able to recognize or bind to PAMPs in order to induce innate immunity. Cells that typically have pattern recognition receptors include macrophages, dendritic cells, endothelial cells, mucosal epithelial cells, and lymphocytes. Many pattern-recognition receptors are located on the surface of these cells where they can interact with PAMPs on the surface of microbes. Others PRRs are found within the phagolysosomes of phagocytes where they can interact with PAMPs located within microbes that have been phagocytosed. Some PRRs are found in the cytosol of the cell.

cathelicidins

proteins produced by skin and mucosal epithelial cells. The two peptides produced upon cleavage of the cathelicidin are directly toxic to a variety of microorganisms. One pepitide also can bind to and neutralize LPS from Gram-negative cell walls to reduce inflammation.

defensins

short cationic peptides 30-40 amino acids long that are directly toxic by disrupting the cytoplasmic membrane of a variety of microorganisms causing leakage of cellular needs. They also activate cells for an inflammatory response. Defensins are produced by leukocytes, epithelial cells, and other cells. They are also found in blood plasma and mucus. Certain defensins also disrupt the envelopes of some viruses.

Briefly describe how the body recognizes an antigen as foreign.

the B-lymphocytes and T-lymphocytes are the cells that carry out the immune responses. The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes and T-lymphocytes by means of epitope-specific receptor molecules having a shape complementary to that of the epitope (similar to interlocking pieces of a puzzle).

List 3 characteristics an antigen must have to be immunogenic.

•be of high molecular weight, •exhibit chemical complexity, and •exhibit foreignness (recognized as non-self by the body).