Microbiology Chapter 16: Innate Immunity

Lyzsozyme

-mucous secreted by esophagus contains this antibacterial enzyme -also in tear, etc. -cleaves bond between NAG and NAM in peptidoglycan, a component of cell wall -more effective vs Gram (+), which lacks protective outer membrane associated with Gram(-)

Innate immunity

-nonspecific -natural, built in -quick/immediate -not perfect nor impenetrable -can cause damage to the body in form of signs/symptoms

List common PAMPs & their defs

-peptidoglycan, found in bacterial cell walls; -flagellin, a protein found in bacterial flagella; -lipopolysaccharide (LPS) from the outer membrane of gram-negative bacteria; -lipopeptides, molecules expressed by most bacteria; and -nucleic acids such as viral DNA or RNA.

Physical Defenses (list them)

-physical barrier -mechanical barrier -microbiome *Physical defenses provide the body's most basic form of nonspecific defense. They include physical barriers to microbes, such as the skin and mucous membranes, as well as mechanical defenses that physically remove microbes and debris from areas of the body where they might cause harm or infection. In addition, the microbiome provides a measure of physical protection against disease, as microbes of the normal microbiota compete with pathogens for nutrients and cellular binding sites necessary to cause infection.

Outcome of complement activation

Although each complement activation pathway is initiated in a different way, they all provide the same protective outcomes: opsonization, inflammation, chemotaxis, and cytolysis. The term opsonization refers to the coating of a pathogen by a chemical substance (called an opsonin) that allows phagocytic cells to recognize, engulf, and destroy it more easily. Opsonins from the complement cascade include C1q, C3b, and C4b. Additional important opsonins include mannose-binding proteins and antibodies. The complement fragments C3a and C5a are well-characterized anaphylatoxins with potent proinflammatory functions. Anaphylatoxins activate mast cells, causing degranulation and the release of inflammatory chemical signals, including mediators that cause vasodilation and increased vascular permeability. C5a is also one of the most potent chemoattractants for neutrophils and other white blood cells, cellular defenses that will be discussed in the next section.

What does bone marrow contains?

Bone marrow contains the hematopoietic stem cells (HSC) that differentiate and mature into the various types of blood cells and lymphocytes (see [link]).

MECHANICAL DEFENSES

In addition to physical barriers that keep microbes out, the body has a number of mechanical defenses that physically remove pathogens from the body, preventing them from taking up residence.

Relationship between PAMPs and PRRs?

In addition to providing the first step of pathogen recognition, the interaction between PAMPs and PRRs on macrophages provides an intracellular signal that activates the phagocyte, causing it to transition from a dormant state of readiness and slow proliferation to a state of hyperactivity, proliferation, production/secretion of cytokines, and enhanced intracellular killing. PRRs on macrophages also respond to chemical distress signals from damaged or stressed cells. This allows macrophages to extend their responses beyond protection from infectious diseases to a broader role in the inflammatory response initiated from injuries or other diseases.

Does the lymphatic system have a normal microbiota? Circulatory system?

Like the circulatory system, the lymphatic system does not have a normal microbiota, and the large numbers of immune cells typically eliminate transient microbes before they can establish an infection. Only microbes with an array of virulence factors are able to overcome these defenses and establish infection in the lymphatic system. However, when a localized infection begins to spread, the lymphatic system is often the first place the invading microbes can be detected.

Lymph nodes

Lymph nodes are bean-shaped organs situated throughout the body. These structures contain areas called germinal centers that are rich in B and T lymphocytes. The lymph nodes also contain macrophages and dendritic cells for antigen presentation. Lymph from nearby tissues enters the lymph node through afferent lymphatic vessels and encounters these lymphocytes as it passes through; the lymph exits the lymph node through the efferent lymphatic vessels (Figure).

Plasma Protein chemical Mediators

Many nonspecific innate immune factors are found in plasma, the fluid portion of blood. Plasma contains electrolytes, sugars, lipids, and proteins, each of which helps to maintain homeostasis (i.e., stable internal body functioning), and contains the proteins involved in the clotting of blood. Additional proteins found in blood plasma, such as acute-phase proteins, complement proteins, and cytokines, are involved in the nonspecific innate immune response.

Acute Phase Proteins -def, expand -examples & functions

The acute-phase proteins are another class of antimicrobial mediators. Acute-phase proteins are primarily produced in the liver and secreted into the blood in response to inflammatory molecules from the immune system. Examples of acute-phase proteins include C-reactive protein, serum amyloid A, ferritin, transferrin, fibrinogen, and mannose-binding lectin. Each of these proteins has a different chemical structure and inhibits or destroys microbes in some way (Table). Some Acute-Phase Proteins and Their Functions *C-reactive protein & Serum Amyloid A -Function: Coats bacteria (opsonization), preparing them for ingestion by phagocytes *Ferritin & Transferrin -Function: Bind and sequester iron, thereby inhibiting the growth of pathogens *Fibrinogen -Function: Involved in formation of blood clots that trap bacterial pathogens *Mannose-binding lectin -Function: Activates complement cascade

Chemical Defenses: how are chemical mediators produced? diff types?

Some chemical mediators are endogenously produced, meaning they are produced by human body cells; others are produced exogenously, meaning that they are produced by certain microbes that are part of the microbiome. Some mediators are produced continually, bathing the area in the antimicrobial substance; others are produced or activated primarily in response to some stimulus, such as the presence of microbes.

Activation of Adaptive Immunity

TLRs bound to PAMPs induce the release of cytokines from innate immune cells that regulate the intensity and duration of immune responses

The Alternative Pathway

The alternative pathway is initiated by the spontaneous activation of the complement protein C3. The hydrolysis of C3 produces two products, C3a and C3b. When no invader microbes are present, C3b is very quickly degraded in a hydrolysis reaction using the water in the blood. However, if invading microbes are present, C3b attaches to the surface of these microbes. Once attached, C3b will recruit other complement proteins in a cascade (Figure).

The Classical Pathway

The classical pathway provides a more efficient mechanism of activating the complement cascade, but it depends upon the production of antibodies by the specific adaptive immune defenses. To initiate the classical pathway, a specific antibody must first bind to the pathogen to form an antibody-antigen complex. This activates the first protein in the complement cascade, the C1 complex. The C1 complex is a multipart protein complex, and each component participates in the full activation of the overall complex. Following recruitment and activation of the C1 complex, the remaining classical pathway complement proteins are recruited and activated in a cascading sequence (Figure).

what moves faster: lymphatic fluids or blood?

blood!! --lymphatic fluids move more slowly than blood b/c they are not pressurized. Open system of fluid moving in one direction toward drainage points into veins just above the heart

Sebum is produced how?

endogenous mediator

PHYSICAL BARRIERS Endothelial cells? types of junctions? Skin layers? Mucous membranes? Mucociliated Escalator? Endothelia?

endothelial cells that line blood vessels have very tight cell-to-cell junctions, blocking microbes from gaining access to the bloodstream. Cell junctions are generally composed of cell membrane proteins that may connect with the extracellular matrix or with complementary proteins from neighboring cells. Invading microorganisms may attempt to break down these substances chemically, using enzymes such as proteases that can cause structural damage to create a point of entry for pathogens. Tight junctions - two membranes connected with many spot welds in multiple lines. Desmosomes - two membranes with long strands weaving them together. Gap junctions - two membranes with a few spot welds each of which has a pore in the center. There are multiple types of cell junctions in human tissue, three of which are shown here. Tight junctions rivet two adjacent cells together, preventing or limiting material exchange through the spaces between them. Desmosomes have intermediate fibers that act like shoelaces, tying two cells together, allowing small materials to pass through the resulting spaces. Gap junctions are channels between two cells that permit their communication via signals. (credit: modification of work by Mariana Ruiz Villareal) *SKIN--skin barrier, which is composed of three layers of closely packed cells. The thin upper layer is called the epidermis. A second, thicker layer, called the dermis, contains hair follicles, sweat glands, nerves, and blood vessels. A layer of fatty tissue called the hypodermis lies beneath the dermis and contains blood and lymph vessels **MUCUS--The epithelial cells secrete a moist, sticky substance called mucus, which covers and protects the more fragile cell layers beneath it and traps debris and particulate matter, including microbes. Mucus secretions also contain antimicrobial peptides. In many regions of the body, mechanical actions serve to flush mucus (along with trapped or dead microbes) out of the body or away from potential sites of infection. For example, in the respiratory system, inhalation can bring microbes, dust, mold spores, and other small airborne debris into the body. This debris becomes trapped in the mucus lining the respiratory tract, a layer known as the mucociliary blanket. The epithelial cells lining the upper parts of the respiratory tract are called ciliated epithelial cells because they have hair-like appendages known as cilia. Movement of the cilia propels debris-laden mucus out and away from the lungs. The expelled mucus is then swallowed and destroyed in the stomach, or coughed up, or sneezed out (Figure). This system of removal is often called the mucociliary escalator. --ENDOTHELIA: The epithelial cells lining the urogenital tract, blood vessels, lymphatic vessels, and certain other tissues are known as endothelia. These tightly packed cells provide a particularly effective frontline barrier against invaders. . . The cell junctions in the blood vessels traveling through the CNS are some of the tightest and toughest in the body, preventing any transient microbes in the bloodstream from entering the CNS. This keeps the cerebrospinal fluid that surrounds and bathes the brain and spinal cord sterile under normal conditions.

Phagocytes

phagocytes—cells whose main function is to seek, ingest, and kill pathogens

Chemical Defenses: The Complement System

*Serum proteins produced by the liver that assist the immune system in destroying microbes -act in a cascade in a process called complement activation *Proteins are designated with uppercase C and numbered in order of discovery -activated fragments are indicated with lowercase a and b *C3a and C3b are the major players in all three complement pathways

Slight acidity of Urine

(pH 6) inhibits growth of many microbes and pathogens that manage to survive the acidic environment of the stomach

Fever

*Abnormally high body temperature -higher temp good for some immune responses -higher temps slow mesophilic bacterial growth *Hypothalamus is normally set at 37C -cytokines released in response to LPS cause hypothalamus to release prostaglandins that reset to a higher temperature --body constricts the blood vessels and shivering occurs (which raises temp) --after infection, vasodilation and sweating occurs and the body temp falls (crisis)

Granulocytes (including neutrophils, eosinophils, and basophils)

*Abundant granules in cytoplasm; nucleus normally lobed *Nonspecific (innate) resistance to disease *Classified according to membrane-bound granules in cytoplasm

Chemical Defenses of Nonspecific Innate Immunity: DEFENSE: Plasma protein mediators

*Acute-phase proteins (C-Reactive protein, serum amyloid A, ferritin, fibrinogen, transferrin, and mannose-binding lectin) -Function: Inhibit the growth of bacteria and assist in trapping and killing of bacteria *Complements C3b & C4b -Function: Opsonization of pathogens to aid phagocytosis *Complement C5a -Function: chemoattractant for phagocytes *Complemetns C3a and C5a -Function: proinflammatory & anaphylatoxins

Chemical Defenses

*Antimicrobial peptides (AMPs) are short peptides produced in response to protein and sugar molecules on microbes -inhibit cell wall synthesis -form pores in plasma membrane *Broad spectrum of activity against bacteria, fungi, and viruses *Acute phase proteins produced in the liver *Limiting an essential nutrient is one method to inhibit microbial growth (colonization & spread). -animals limit iron available in body fluids by binding it to transport/storage proteins EX: --Transferrin: found in blood and tissue fluids --Lactoferrin: found in milk, saliva, and mucous --Ferritin: found in the liver, spleen, and red bone marrow --Hemoglobin: located in RBCs -Bacteria produce siderophore proteins to compete with host iron-binding proteins to scavenge iron

Characteristics of Selected Antimicrobial Peptides (AMPs) *Bacteriocins *Cathelidin *Defensins *Dermicidin *Histatins AMP--Secreted by--Body site--Pathogen inhibited--Mode of action

*Bacteriocins -selected by: resident microbiota -body site: Gastrointestinal tract -pathogens inhibited: bacteria -mode of action: Disrupts membrane *Cathelicidin -selected by: Epithelial cells, macrophages, and other cell types -body site: Skin -pathogens inhibited: Bacteria & fungi -mode of action: Disrupts membrane *Defensins -selected by: Epithelial cells, macrophages, neutrophils -body site: throughout the body -pathogens inhibited: fungi, bacteria and many viruses -mode of action: disrupt membrane *Dermicidin -selected by: Sweat glands -body site: Skin -pathogens inhibited: Bacteria and Fungi -mode of action: disrupts membrane integrity and ion channels *Histatins -selected by: Salivary glands -body site: Oral cavity -pathogens inhibited: Fungi -mode of action: Disrupts intracellular function

Physical Defenses of Nonspecific Innate Immunity -List them, examples, functions

*Cellular (physical) barriers -EX: Skin, mucous membranes, endothelial cells -Function:Deny entry to pathogens *Mechanical defenses -EX: Shedding of skin cells, mucociliary sweeping, peristalsis, flushing action of urine and tears -Functions: Remove pathogens from potential sites of infection *Microbiome -EX: Resident bacteria of the skin, upper respiratory tract, gastrointestinal tract, and genitourinary tract -Functions: Compete with pathogens for cellular binding sites and nutrients

Platelets

*Cellular fragments surrounded by a plasma membrane and containing granules; stains purple *Hemostasis; release growth factors for repair and healing of tissue *formed from megakaryocytes that remain in the red bone marrow and shed platelets into circulation

Phagocytosis

*Chemotaxis: signals attract phagocytes to microbes *Adherence: attachment of a phagocyte to the surface of the microbe *Ingestion: Opsonization = microbe is coated with serum proteins, making ingestion easier *Digestion: Microbe is digested inside a phagolysosome

Chronic Inflammation

*Chronic inflammation may lead to the formation of granulomas, pockets of infected tissue walled off and surrounded by WBCs. Macrophages and other phagocytes wage an unsuccessful battle to eliminate the pathogens and dead cellular materials within a granuloma. --One example of a disease that produces chronic inflammation is tuberculosis, which results in the formation of granulomas in lung tissues. A tubercular granuloma is called a tubercle (Figure). Tuberculosis will be covered in more detail in Bacterial Infections of the Respiratory Tract. *Chronic inflammation is not just associated with bacterial infections. Chronic inflammation can be an important cause of tissue damage from viral infections. The extensive scarring observed with hepatitis C infections and liver cirrhosis is the result of chronic inflammation.

Cytokines -what are they? -expand -Functions & defs -classes & defs

*Cytokines are soluble proteins that act as communication signals between cells. In a nonspecific innate immune response, various cytokines may be released to stimulate production of chemical mediators or other cell functions, such as cell proliferation, cell differentiation, inhibition of cell division, apoptosis, and chemotaxis. *When a cytokine binds to its target receptor, the effect can vary widely depending on the type of cytokine and the type of cell or receptor to which it has bound. The function of a particular cytokine can be described as autocrine, paracrine, or endocrine (Figure). *In autocrine function, the same cell that releases the cytokine is the recipient of the signal; in other words, autocrine function is a form of self-stimulation by a cell. *In contrast, paracrine function involves the release of cytokines from one cell to other nearby cells, stimulating some response from the recipient cells. *Last, endocrine function occurs when cells release cytokines into the bloodstream to be carried to target cells much farther away. *Three important classes of cytokines are the interleukins, chemokines, and interferons. *The interleukins were originally thought to be produced only by leukocytes (white blood cells) and to only stimulate leukocytes, thus the reasons for their name. Although interleukins are involved in modulating almost every function of the immune system, their role in the body is not restricted to immunity. Interleukins are also produced by and stimulate a variety of cells unrelated to immune defenses. *The chemokines are chemotactic factors that recruit leukocytes to sites of infection, tissue damage, and inflammation. In contrast to more general chemotactic factors, like complement factor C5a, chemokines are very specific in the subsets of leukocytes they recruit. *Interferons are a diverse group of immune signaling molecules and are especially important in our defense against viruses. Type I interferons (interferon-α and interferon-β) are produced and released by cells infected with virus. These interferons stimulate nearby cells to stop production of mRNA, destroy RNA already produced, and reduce protein synthesis. These cellular changes inhibit viral replication and production of mature virus, slowing the spread of the virus. Type I interferons also stimulate various immune cells involved in viral clearance to more aggressively attack virus-infected cells. Type II interferon (interferon-γ) is an important activator of immune cells (Figure).

Chemical Defenses: Complement Activation

*Cytolysis -activated complement create a membrane attack complex (MAC) -opsonization: promotes attachment of phagocyte to a microbe -Inflammation: activated complement proteins bind to mast cells, releasing histamine *Regulation of complement -regulation of complement: regulatory proteins readily break down complement proteins, minimizing host cell destruction -complement and disease: lack of complement proteins causes susceptibility to infections -evading the complement system: bacterial capsules prevent complement activation

Chemical defenses of Nonspecific Innate Immunity: DEFENSE: Antimicrobial Peptides

*Defensins, bacteriocins, dermicidin, cathelidin, histatins -Function: kill bacteria by attacking membrane or interfering with cell functions

Extravasation (diapedesis)

*Definition = To reach pathogens located in infected tissue, leukocytes must pass through the walls of small capillary blood vessels within tissues. This process, called extravasation, or diapedesis, is initiated by complement factor C5a, as well as cytokines released into the immediate vicinity by resident macrophages and tissue cells responding to the presence of the infectious agent (Figure). *Note that extravasation does not occur in arteries or veins. These blood vessels are surrounded by thicker, multilayer protective walls, in contrast to the thin single-cell-layer walls of capillaries. Furthermore, the blood flow in arteries is too turbulent to allow for rolling adhesion. Also, some leukocytes tend to respond to an infection more quickly than others. The first to arrive typically are neutrophils, often within hours of a bacterial infection. By contract, monocytes may take several days to leave the bloodstream and differentiate into macrophages. *Steps -(1) leukocytes in the blood respond to chemical attractants released by pathogens and chemical signals from nearby injured cells. -(2) the leukocytes squeeze between the cells of the capillary wall as they follow the chemical signals to where they are most concentrated (positive chemotaxis). -(3) Within the damaged tissue, neutrophils release chemicals that break apart pathogens. Monocytes differentiate into macrophages. Neutrophils and macrophages phagocytize pathogens and cellular debris.

Chemical Defenses: The Lectin Pathway

*Does not require antibodies! *macrophages ingest pathogens, releasing cytokines that stimulate lectin production in the liver *mannose-binding lectin (MBL) binds to mannose, activating C2, C4 *C2a & C4b activate C3, which functions the same as in the classical and alternative pathways

Chemical Defenses of Nonspecific Innate Immunity: DEFENSE: Inflammation-eliciting mediators

*Histamine -promotes vasodilation, bronchoconstriction, smooth muscle contraction, increased secretion and mucus production *Leukotrienes -promote inflammation; stronger and longer lasting than histamine *Prostaglandins -promote inflammation and fever *Bradykinin -increases vasodilation and vascular permeability, leading to edema

Inflammation

*Inflammation is the body's local response to damage *Functions of Inflammation: -destroy injurious agent -limit injurious agent effects on the body -repair and replace tissues damaged by the injurious agent *5 signs and symptoms of inflammation -1-Erythema (redness) -2-Edema (swelling) -3-Heat -4-Pain -5-Altered function *Acute inflammation is the body's immediate local response to damage -1st is vasoconstriction for blood loss -2nd is vasodilation and increased vascular permeability by histamine released from damaged cells -3rd is influx of phagocytes that will recognize PAMPs *IMAGE -1. leukocytes in the blood respond to chemical attractants released by pathogens and chemical signals from nearby injured cells -2. the leukocytes squeeze between the cells of the capillary wall as they follow the chemical signals to where they are most concentrated (positive chemotaxis) -3. Within the damaged tissue, neutrophils release chemicals that break apart pathogens. Monocytes differentiate into macrophages. Neutrophils and macrophages phagocytize pathogens and cellular debris

Microbial Evasion of Phagocytosis

*Inhibit adherence: M protein, capsules -Streptococcus pyrogenes, Streptococcus pneumoniae *Kill phagocytes: leukocidins -S. aureus *Lyse phagocytes: membrane attack copmplex -Listeria monocytogenes *Escape phagosome -Shigella spp., Rickettsia spp. *Prevent phagosome: lysosome fusion -Mycobacterium tuberculosis, HIV *Survive in phagosome -Coxiella burnetii

Chemical Defenses

*Interferons are antiviral cytokines produced by host cells -Type I (IFN-alpha and IFN-beta): produced by infected cells cause by neighboring host cells to produce antiviral proteins (AVPs) that inhibit viral replication (stop RNA production) -Type II (IFN-gamma): causes neutrophils and macrophages to kill bacteria

Chemical Defenses of Nonspecific Innate Immunity: DEFENSE: Cytokines

*Interleukins -Stimulate and modulate most functions of immune system *Chemokines -recruit WBCs to infected area *Interferons -alert cells to viral infected cells, induce apoptosis of viral-infected cells, induce antiviral defenses in infected and nearby uninfected cells, stimulate immune cells to attack virus-infected cells

Agranulocytes (including lymphocytes and monocytes)

*Lack abundant granules in cytoplasm; have a simple-shaped nucleus that may be indented *Body defenses *Group consists of two major cell types from different lineages

Monocytes

*Largest leukocyte; has an indented or horseshoe-shaped nucleus *very effective phagocytic cells engulfing pathogens or worn-out cells; also serve as antigen-presenting cells (APCs) or other components of the immune system *produced in red bone marrow; referred to as macrophages and dendritic cells after leaving the circulation ****The monocytes differentiate into macrophages and dendritic cells, which are collectively referred to as the mononuclear phagocyte system. ** They are particularly important residents of lymphoid tissue, as well as nonlymphoid sites and organs. Macrophages and dendritic cells can reside in body tissues for significant lengths of time. **Macrophages in specific body tissues develop characteristics suited to the particular tissue. Not only do they provide immune protection for the tissue in which they reside but they also support normal function of their neighboring tissue cells through the production of cytokines. Macrophages are given tissue-specific names, and a few examples of tissue-specific macrophages are listed in Table. **Dendritic cells are important sentinels residing in the skin and mucous membranes, which are portals of entry for many pathogens. Monocytes, macrophages, and dendritic cells are all highly phagocytic and important promoters of the immune response through their production and release of cytokines. These cells provide an essential bridge between innate and adaptive immune responses, as discussed in the next section as well as the next chapter.

Phagocyte migration and phagocytosis

*Margination is the sticking of phagocytes to blood vessels in response to cytokines at the site of inflammation *Phagocytes squeeze between endothelial cells of blood vessels via diapedesis *IMAGE -Margination = phagocytes stick to endothelium -Diapedesis = phagocytes squeeze between endothelial cells -Phagocytosis = of invading bacterial occurs *Note: Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall.

Mast Cells

*Mast cells function similarly to basophils but can be found in tissues outside the bloodstream. **Hematopoiesis also gives rise to mast cells, which appear to be derived from the same common myeloid progenitor cell as neutrophils, eosinophils, and basophils. **Functionally, mast cells are very similar to basophils, containing many of the same components in their granules (e.g., histamine) and playing a similar role in allergic responses and other inflammatory reactions. **However, unlike basophils, mast cells leave the circulating blood and are most frequently found residing in tissues. They are often associated with blood vessels and nerves or found close to surfaces that interface with the external environment, such as the skin and mucous membranes in various regions of the body

Natural Killer Cells

*Natural killer (NK) cells are lymphocytes that recognize and kill abnormal or infected cells by releasing proteins that trigger apoptosis. **Most lymphocytes are primarily involved in the specific adaptive immune response, and thus will be discussed in the following chapter. **An exception is the natural killer cells (NK cells); these mononuclear lymphocytes use nonspecific mechanisms to recognize and destroy cells that are abnormal in some way. **(1) Cancer cells and (2) cells infected with viruses are two examples of cellular abnormalities that are targeted by NK cells. Recognition of such cells involves a complex process of identifying inhibitory and activating molecular markers on the surface of the target cell. Molecular markers that make up the major histocompatibility complex (MHC) are expressed by healthy cells as an indication of "self." This will be covered in more detail in next chapter. NK cells are able to recognize normal MHC markers on the surface of healthy cells, and these MHC markers serve as an inhibitory signal preventing NK cell activation. However, cancer cells and virus-infected cells actively diminish or eliminate expression of MHC markers on their surface. When these MHC markers are diminished or absent, the NK cell interprets this as an abnormality and a cell in distress. This is one part of the NK cell activation process (Figure). NK cells are also activated by binding to activating molecular molecules on the target cell. These activating molecular molecules include "altered self" or "nonself" molecules. When a NK cell recognizes a decrease in inhibitory normal MHC molecules and an increase in activating molecules on the surface of a cell, the NK cell will be activated to eliminate the cell in distress. **Once a cell has been recognized as a target, the NK cell can use several different mechanisms to kill its target. For example, it may express cytotoxic membrane proteins and cytokines that stimulate the target cell to undergo apoptosis, or controlled cell suicide. NK cells may also use perforin-mediated cytotoxicity to induce apoptosis in target cells. This mechanism relies on two toxins released from granules in the cytoplasm of the NK cell: (1) perforin, a protein that creates pores in the target cell, and (2) granzymes, proteases that enter through the pores into the target cell's cytoplasm, where they trigger a cascade of protein activation that leads to apoptosis. The NK cell binds to the abnormal target cell, releases its destructive payload, and detaches from the target cell. While the target cell undergoes apoptosis, the NK cell synthesizes more perforin and proteases to use on its next target. NK cells contain these toxic compounds in granules in their cytoplasm. When stained, the granules are azurophilic and can be visualized under a light microscope (Figure). Even though they have granules, NK cells are not considered granulocytes because their granules are far less numerous than those found in true granulocytes. Furthermore, NK cells have a different lineage than granulocytes, arising from lymphoid rather than myeloid stem cells (Figure).

Basophils

*Nucleus generally two-lobed but difficult to see due to presence of heavy, dense, dark purple granules *Pro-inflammatory *Least common leukocyte; lifespan unknown **granules' able to absorb the basic dye methylene blue (Figure). Their stimulation and degranulation can result from multiple triggering events. Activated complement fragments C3a and C5a, produced in the activation cascades of complement proteins, act as anaphylatoxins by inducing degranulation of basophils and inflammatory responses. This cell type is important in allergic reactions and other responses that involve inflammation. **One of the most abundant components of basophil granules is histamine, which is released along with other chemical factors when the basophil is stimulated. These chemicals can be chemotactic and can help to open the gaps between cells in the blood vessels. Other mechanisms for basophil triggering require the assistance of antibodies, as discussed in B Lymphocytes and Humoral Immunity.

Eosinophils

*Nucleus generally two-lobed; bright red-orange granules *phagocytic cells; particularly effective with antigen-antibody complexes; release antihistamines; combat parasitic infections *Lifespan of minutes to days **Eosinophils are granulocytes that protect against protozoa and helminths; they also play a role in allergic reactions. The granules of eosinophils, which readily absorb the acidic reddish dye eosin, contain histamine, degradative enzymes, and a compound known as major basic protein (MBP) (Figure). MBP binds to the surface carbohydrates of parasites, and this binding is associated with disruption of the cell membrane and membrane permeability.

Neutrophils

*Nucleus loves increase with age; pale lilac granules *phagocytic; particularly effective against bacteria; release cytotoxic chemicals from granules *most common leukocyte; lifespan of minutes to days **The neutrophils, also called polymorphonuclear neutrophils (PMNs), have a nucleus with three to five lobes and small, numerous, lilac-colored granules. **Neutrophils (PMNs) are frequently involved in the elimination and destruction of extracellular bacteria. They are capable of migrating through the walls of blood vessels to areas of bacterial infection and tissue damage, where they seek out and kill infectious bacteria. PMN granules contain a variety of defensins and hydrolytic enzymes that help them destroy bacteria through phagocytosis (described in more detail in Pathogen Recognition and Phagocytosis) In addition, when many neutrophils are brought into an infected area, they can be stimulated to release toxic molecules into the surrounding tissue to better clear infectious agents. This is called degranulation. **As neutrophils fight an infection, a visible accumulation of leukocytes, cellular debris, and bacteria at the site of infection can be observed. This buildup is what we call pus (also known as purulent or suppurative discharge or drainage). The presence of pus is a sign that the immune defenses have been activated against an infection; a small amount of pus formation can indicate a strong immune response, artificially inducing pus formation does not promote recovery.

What's an example of one group of PRRs? -where are they located?

*One group of PRRs is the toll-like receptors (TLRs), which bind to various PAMPs and communicate with the nucleus of the phagocyte to elicit a response. *Many TLRs (and other PRRs) are located on the surface of a phagocyte, but some can also be found embedded in the membranes of interior compartments and organelles (Figure). These interior PRRs can be useful for the binding and recognition of intracellular pathogens that may have gained access to the inside of the cell before phagocytosis could take place. Viral nucleic acids, for example, might encounter an interior PRR, triggering production of the antiviral cytokine interferon.

How can PRRs aid in phagocytosis

*PRRs can aid in phagocytosis by first binding to the pathogen's surface, but phagocytes are also capable of engulfing nearby items even if they are not bound to specific receptors. To engulf the pathogen, the phagocyte forms a pseudopod that wraps around the pathogen and then pinches it off into a membrane vesicle called a phagosome. **Acidification of the phagosome (pH decreases to the range of 4-5) provides an important early antibacterial mechanism. The phagosome containing the pathogen fuses with one or more lysosomes, forming a phagolysosome. **Formation of the phagolysosome enhances the acidification, which is essential for activation of pH-dependent digestive lysosomal enzymes and production of hydrogen peroxide and toxic reactive oxygen species. Lysosomal enzymes such as lysozyme, phospholipase, and proteases digest the pathogen. Other enzymes are involved a respiratory burst. During the respiratory burst, phagocytes will increase their uptake and consumption of oxygen, but not for energy production. The increased oxygen consumption is focused on the production of superoxide anion, hydrogen peroxide, hydroxyl radicals, and other reactive oxygen species that are antibacterial.

Pathogen Recognition

*Pathogen-associated molecular patterns (PAMPs) are molecules associated with pathogens and are recognized by phagocytes -Peptidoglycan -flagellin -LPS -Lipopeptides -Viral DNA or RNA *Toll-like receptors (TLRs) on phagocytes attach to PAMPs

Phagocytes

*Phago: from the Greek, meaning eat *Cyte: from the Greek, cell *Many types of phagocytes with specializations *Fixed macrophages are residents in tissues and organs *Free (wandering) macrophages roam tissues and gather at sites of infection

Innate Immunity: Overview

*Physical Factors -skin -mucous membranes -normal flora *Chemical Factors -sebum lysozyme -cerumen (earwax) -Saliva -gastric juices -vaginal secretions -urine -antimicrobial peptides -iron-binding proteins -complement -interferons *WBCs -granulocytes -agranulocytes *Inflammation & fever *FROM PICTURE -lyzozyme in tears and other secretions dissolves cell walls normal flora compete with pathogens -skin is a physical barrier, produces antimicrobial fatty acids and its normal flora inhibit pathogen colonization -rapid pH change inhibits microbial growth -flushing of urinary tract prevents colonization -removal of particles including microbes by rapid passage of air over cilia in nasopharynx -mucous, cilia lining the trachea suspend and move microbes out of body -blood proteins inhibit microbial growth -mucus and phagocytes in lungs prevent colonization -stomach acidity (pH 2) -normal flora compete with pathogens

Chemical Defenses: The Classical Pathway

*Requires antibodies *Antibodies bind to antigens activating C1 *C1 splits and activates C2 and C4 *C2a and C4b combine and activate C3 -C3a functions in inflammation -C3b functions in cytolysis and opsonization

Chemical defenses of Nonspecific Innate Immunity: DEFENSE: Chemicals and enzymes in body fluids --these EX and their functions *Sebum *oleic acid *lysozyme *acid *Digestive enzymes and bile *lactoferrin and transferrin *surfactant in lungs

*Sebum from sebaceous glands -provides oil barrier protecting hair follicle pores from pathogens *Oleic acid from sebum and skin microbiota -lowers pH to inhibit pathogens *Lysozyme in secretions -kills bacteria by attacking cell wall *Acid in stomach, urine, and vagina -inhibits or kills bacteria *Digestive enzymes & bile -kill bacteria *Lactoferrin & transferrin -bind and sequester iron, inhibiting bacterial growth *Surfactant in lungs -kills bacteria

Physical defenses

*Skin -Epidermis: outer portion made of tightly packed epithelial cells containing keratin, a protective protein -Dermis: inner portion made of connective tissue -Shedding and drying of skin inhibits microbial growth *Mucous Membranes -epithelial layer that lines the gastrointestinal, respiratory and genitourinary tracts -mucous: viscous glycoproteins that trap microbes and prevent tracts from drying out -lacrimal apparatus: drains tears, washes eyes *Ciliary escalator transports microbes trapped in mucous out of lungs *ear wax prevents microbes from entering the inner ear *Urine cleans the urethra *Peristalsis, defecation, vomiting, diarrhea *Normal microbiota compete with pathogens via microbial antagonism -produce substances harmful to pathogens -alter conditions that affect pathogen survival *Commensalism: one organism benefits while the other (host) is unharmed *Probiotics: live microbial cultures administered to exert a beneficial effect *Sebum forms a protective film and lowers the pH (3 - 5) of skin *Lysozyme in perspiration, tears, saliva, and urine destroys bacterial cells walls *Low pH (1.2 - 3.0) of gastric juices destroys bacteria and toxins *Low pH (3 - 5) of vaginal secretions inhibit microbes

Lymphocytes

*Spherical cells with a single, often large nucleus occupying much of the cell's volume; stains purple; seen in large (natural killer cells) and small (B & T cells) variants

Membrane Attack Complex (MAC) -expand -what is MAC effect against

*The complement proteins C6, C7, C8, and C9 assemble into a membrane attack complex (MAC), which allows C9 to polymerize into pores in the membranes of gram-negative bacteria. These pores allow water, ions, and other molecules to move freely in and out of the targeted cells, eventually leading to cell lysis and death of the pathogen (Figure). *However, the MAC is only effective against gram-negative bacteria; it cannot penetrate the thick layer of peptidoglycan associated with cell walls of gram-positive bacteria. Since the MAC does not pose a lethal threat to gram-positive bacterial pathogens, complement-mediated opsonization is more important for their clearance.

Types of lymphoid tissues in lymphatic system?

*The lymphatic system contains two types of lymphoid tissues. 1. The primary lymphoid tissue includes bone marrow and the thymus. 2. The secondary lymphoid tissues include the spleen, lymph nodes, and several areas of diffuse lymphoid tissues underlying epithelial membranes.

Three major functions of the lymphatic system are

*These fluids, termed lymph, also contain large numbers of white blood cells. *The three major functions of the lymphatic system are: (1) to collect excess fluid from the inter-cellular spaces, (2) to transport digested fats to the circulatory system, and (3) to provide innate and adaptive defense against infections.

Cellular Defenses

*WBC count measures leukocytes in the blood -high WBC counts may indicate bacterial infections, autoimmune diseases, or side effects of medications -low WBC counts may indicate viral infections, pneumonia, autoimmune diseases, or cancers *Immunity: ability to ward off disease *Susceptibility: lack of resistance to a disease *Cells and cell fragments suspended in plasma -Erythrocytes (RBCs) -Leukocytes (WBCs) -Platelets *Created in red bone marrow stem cells via hematopoiesis *Granulocytes are leukocytes with granules in their cytoplasm that are visible with a light microscope -Neutrophils: phagocytic, work in early stages of infection -Basophils and Mast Cells: release histamine; work in allergic responses -Eosinophils: phagocytic, produce toxic proteins against parasites and helminths *Agranulocytes are leukocytes with granules in their cytoplasm that are not visible with a light microscope -monocytes: mature into macrophages in tissues where they are phagocytic -dendritic cells: found in the skin, mucous membranes, and thymus; phagocytic -lymphocytes: --NK cells produce perforin and granzymes to kill infected and cancerous host cells --T cells and B cells play a role in adaptive immunity (next lecture)

Chemical Defenses: The Alternative Pathway

*does not require antibodies! *Factors B, D, & P bind to microbe *C3 combines with factors B, D, and P *C3 splits into C3a & C3b, functioning the same as the classical pathway

The lymphatic systm

*lymph, lymphatic vessels, lymphoid tissue and red bone marrow *contains lymphocytes and phagocytic cells *lymph carries microbes to lymph nodes where lymphocytes and macrophages destroy the pathogen

Internal defenses

-Phagocytic cells -antimicrobial proteins -inflammatory response -Natural killer cells

External defenses

-Skin -Mucous Membranes -Secretions

Two branches of the immune system + descriptions

1. Innate Immunity: nonspecific responses present before exposure to an agent. **Includes inflammatory response & fever. 2. Adaptive Immunity: develops after exposure to an agent and has memory. **Includes Humoral response that produces antibodies and binds to targets & Cell-mediated response that directly destroys targets.

Hematopoiesis

= blood cell formation *All of the formed elements of blood are derived from pluripotent hematopoietic stem cells (HSCs) in the bone marrow. *As the HSCs make copies of themselves in the bone marrow, individual cells receive different cues from the body that control how they develop and mature. As a result, the HSCs differentiate into different types of blood cells that, once mature, circulate in peripheral blood. This process of differentiation, called hematopoiesis, is shown in more detail in Figure. *In terms of sheer numbers, the vast majority of HSCs become erythrocytes. *Much smaller numbers become leukocytes and platelets. *Leukocytes can be further subdivided into granulocytes, which are characterized by numerous granules visible in the cytoplasm, and agranulocytes, which lack granules. Figure provides an overview of the various types of formed elements, including their relative numbers, primary function, and lifespans.

Antimicrobial Peptides (AMPs)

= special class of nonspecific cell-derived mediators with broad spectrum of antimicrobial properties. *AMPs may induce cell damage in microorganisms in a variety of ways, including by inflicting damage to membranes, destroying DNA and RNA, or interfering with cell-wall synthesis. Depending on the specific antimicrobial mechanism, a particular AMP may inhibit only certain groups of microbes (e.g., gram-positive or gram-negative bacteria) or it may be more broadly effective against bacteria, fungi, protozoa, and viruses. Many AMPs are found on the skin, but they can also be found in other regions of the body.

nonfluid portion of blood = formed elements -expand -types & defs

In the previous section, we discussed some of the chemical mediators found in plasma, the fluid portion of blood. The nonfluid portion of blood consists of various types of formed elements, so called because they are all formed from the same stem cells found in bone marrow. The three major categories of formed elements are: red blood cells (RBCs), also called erythrocytes; platelets, also called thrombocytes; and white blood cells (WBCs), also called leukocytes. Red blood cells are primarily responsible for carrying oxygen to tissues. Platelets are cellular fragments that participate in blood clot formation and tissue repair. Several different types of WBCs participate in various nonspecific mechanisms of innate and adaptive immunity. In this section, we will focus primarily on the innate mechanisms of various types of WBCs.

Why are lymphoid tissues considered part of the innate defenses?

It is important to understand that lymphoid tissues contribute to the innate defenses because it contains cells that can phagocytize microorganisms and foreign material.

What happens after degradation?

Once degradation is complete, leftover waste products are excreted from the cell in an exocytic vesicle. However, it is important to note that not all remains of the pathogen are excreted as waste. Macrophages and dendritic cells are also antigen-presenting cells involved in the specific adaptive immune response. These cells further process the remains of the degraded pathogen and present key antigens (specific pathogen proteins) on their cellular surface. This is an important step for stimulation of some adaptive immune responses, as will be discussed in more detail in the next chapter.

Phagocytosis

Once pathogen recognition and attachment occurs, the pathogen is engulfed in a vesicle and brought into the internal compartment of the phagocyte in a process called phagocytosis (Figure).

Mechanism of phagocytosis

Pathogen-associated molecular patterns (PAMPS) are molecules associated with groups of pathogens, that are recognized by the cells of the innate immune system

plasma vs serum

Plasma has clotting factors serum doesn't There are two terms for the fluid portion of blood: plasma and serum. How do they differ if they are both fluid and lack cells? The fluid portion of blood left over after coagulation (blood cell clotting) has taken place is serum.

Erythrocytes

RBCs *Flattened biconcave disk; no nucleus; pale red *Transport oxygen and some CO2 between tissue and lungs *Lifespan abt 120 days

Innate Immunity

Rapid response to a broad range of microbes (PRIMITIVE)

The complement system

The complement system is a group of plasma protein mediators that can act as an innate nonspecific defense while also serving to connect innate and adaptive immunity (discussed in the next chapter). The complement system is composed of more than 30 proteins (including C1 through C9) that normally circulate as precursor proteins in blood. These precursor proteins become activated when stimulated or triggered by a variety of factors, including the presence of microorganisms. Complement proteins are considered part of innate nonspecific immunity because they are always present in the blood and tissue fluids, allowing them to be activated quickly. Also, when activated through the alternative pathway (described later in this section), complement proteins target pathogens in a nonspecific manner.

one bad possible effect of fever

The inflammatory response to bacterial superantigens is one scenario in which a life-threatening fever may develop. Superantigens are bacterial or viral proteins that can cause an excessive activation of T cells from the specific adaptive immune defense, as well as an excessive release of cytokines that overstimulates the inflammatory response. For example, Staphylococcus aureus and Streptococcus pyogenes are capable of producing superantigens that cause toxic shock syndrome and scarlet fever, respectively. Both of these conditions can be associated with very high, life-threatening fevers in excess of 42 °C (108 °F).

The Lectin Pathway

The lectin activation pathway is similar to the classical pathway, but it is triggered by the binding of mannose-binding lectin, an acute-phase protein, to carbohydrates on the microbial surface. Like other acute-phase proteins, lectins are produced by liver cells and are commonly upregulated in response to inflammatory signals received by the body during an infection (Figure).

Complement Activation

The process by which circulating complement precursors become functional is called complement activation. This process is a cascade that can be triggered by one of three different mechanisms, known as the alternative, classical, and lectin pathways.

Spleen

The spleen, an encapsulated structure, filters blood and captures pathogens and antigens that pass into it (Figure). The spleen contains specialized macrophages and dendritic cells that are crucial for antigen presentation, a mechanism critical for activation of T lymphocytes and B lymphocytes (see Major Histocompatibility Complexes and Antigen-Presenting Cells).

What are Pattern recognition receptors?

The structures that allow phagocytic cells to detect PAMPs are called pattern recognition receptors (PRRs).

Thymus

The thymus, a multilobed gland, is located below the sternum and is directly connected with the adaptive defense system considering it is the site of maturation for T cells. We will talk more about this on the adaptive defenses chapter, but it is interesting to notice that the thymus reduces its size as we age, and is replaced by connective tissue and fats.

Leukocytes

WBCs *Obvious dark-staining nucleus *all function in body defenses *Exit capillaries and move into tissues; lifespan of usually a few hours or days

MICROBIOME

resident microbiota

Adaptive Immunity

slower response to specific microbes ADVANCED -Types: *Humoral Response (antibodies) *Cell-mediated response (cytotoxic lympthocytes