Immunology: Chapter 16

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis H. Treatment and prevention of allergy 2. Therapy to counteract allergies b. Desensitization or hyposensitization

-70% of allergic patients benefit from controlled injections of specific allergens as determined by skin tests -This technique, called desensitization or hyposensitization, is a therapeutic way to prevent reactions between allergen, IgE, and mast cells -Allergen preparations contain pure suspensions of plant antigens, venoms, dust mites, dander, and molds -IgG "blocking antibodies" can remove allergen from the system before it can bind to IgE and trigger mast cell degranulation -Allergen delivered in this fashion combines with the IgE itself and takes it from circulation before it can react with the mast cells

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System A. Primary Immunodeficiency Diseases 2. Clinical deficiencies in T-cell development or expression

-A genetic defect in T-cell development results in a broad spectrum of diseases, including severe opportunistic infections, wasting, and canccer -A defective T-cell line is usually more devastating than a defective B-cell line because T helper cells are required to assist in most specific immune reactions DiGeorge Syndrome -Comes about due to errors during embryogenesis or deletions in chromosome 22 -Characterized by a lack of cell-mediated immunity, making children highly susceptible to persistent infections by fungi, protozoa, and viruses -Vaccinations using live, attenuated microbes pose a danger, and common childhood infections such as chickenpox, measles, and mumps in children

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms 4. Platelet-activating Factor

-A lipid released by basophils, neutrophils, monocytes, and macrophages -Stimulation by this factor is similar to that of histamine, including increased vascular permeability, pulmonary smooth muscle contraction, pulmonary edema, hypotension, and a wheal-and-flare response in the skin

16.5 Type IV Hypersensitivities: Cell-Mediated (Delayed) Reactions A. Delayed-type hypersensitivity 1. Infectious Allergy a. Mycobacterium tubercuolosis b. Tuberculin reaction

-A person sensitized by tuberculosis infection is injected with an extract of the bacterium Mycobacterium tuberculosis -An acute skin inflammation at the injcetion site -Arises from time-consuming cellular events involving a specific class of T cells that receive events involving a specific class of T cells (TH1) that receive the processed allergens from dendritic cells -Activated TH cells release cytokines that recruit various inflammatory cells such as macrophages, neutorphils, and eosinophils -Build up of fluid cells at the site gives rise to a red papule

16.3 Type II Hypersensitivities: Reactions That Lyse Foreign Cells D. The Rh Factor and Its Clinical Importance

-A person's Rh type results from a combination of two possible alleles-a dominant one that codes for the factor and a recessive one that does not -A person inheriting at least one Rh gene will be Rh-positive(Rh+); only those persons inheriting two recessive genes are Rh-negative (Rh-) -The "+" or "-" appearing after a blood type (i.e. O+) reflects the Rh status of the person 1. Hemolytic disease of the newborn and Rh incompatibility: -The first Rh+ child is usually not affected because the process begins so late in pregnancy that the child is born before maternal sensitization is completed -In the next pregnancy with an Rh+ fetus, fetal blood cells escape into the maternal circulation late in pregnancy and elicit a memory response -Erythroblastosis fetalis - reflects the release of immature nucleated RBCs into the blood to compensate for destroyed RBCs 2. Preventing Hemolytic Disease of the newborn -Rh0 - Antiserum containing antibodies against the Rh factor. - The antiserum reacts with any fetal RBCs that have escaped into the maternal circulation, thereby preventing the sensitization of the mother's immune system to Rh factor

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System A. Primary Immunodeficiency Diseases 1. Clinical Deficiencies in B-cell development or expression

-A significant number of B-cell deficiencies are X-linked recessive traits, meaning that the gene occurs on the X chromosome and the disease appears primarily in male children Agammaglobulinemia -Means the absence of gamma globulin, the fraction of serum that contains immunoglobulins Hypogammaglobulinemia -Very rare for Ig to be completely absent -T-cell function in these patients is usually normal -The symptoms of recurrent, serious bacterial infections appear

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis C. Mechanisms of Type I allergy: Sensitization and Provocation 3. The Second Contact with Allergen

-After sensitization, the IgE-primed mast cells can remain in the tissues for years -A person can retain the capacity to react immediately upon reexposure -The next time allergen molecules contact these sensitized cells, they bind across adjacent receptors and stimulate degranulation -As chemical mediators are released, they diffuse into the tissues and bloodstream

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis

-Allergic individuals are acutely sensitive to repeated contact with antigens, called allergens, that do not noticeably affect nonallergic individuals. -Similar physiological mechanism, are immediate in onset, and are associated with exposure to specific antigens -Two levels of severity: 1. Atopy - is any chronic local allergy such as hay gever or asthma 2. Anaphylaxis - is a systemic-sometimes fatal-reaction that involves airway obstruction and circulatory collapse

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis A. Who is affected and how?

-Allergies exert profound medical and economic impact -40% of the world's population suffers from one or more allergic conditions -In the US, nearly half of the population is affected by airborne allergens -The predisposition for Type I allergies has a strong familial association. Be aware that what is hereditary is a generalized Susceptibility -The "hygiene hypothesis" - provides one explanation for the occurrence of allergies. A combination of being delivered C section and a maternal history of allergy elevates the risk that a child will be allergic to foods by a factor of 8

16.1 The Immune Response: A Two-Sided Coin B. Overreactions to Antigens: Allergy/Hypersensitivity

-Allergy - refers to an exaggerated immune response that is manifested by inflammation. -Hypersensitivity: Four Types 1. Type I - "common" allergy and anaphylaxis 2. Type II - IgG- and IgM-mediated cell damage 3. Type III - Immune Complex 4. Type IV - Delayed Hypersensitivity -Exogenous Antigens - originating from outside the body (microbes, pollen grains, and foreign cells and proteins) -Endogenous Antigens - Arising from self tissue (autoimmunities)

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis H. Treatment and prevention of allergy 2. Therapy to counteract allergies a. Antihistamines

-Antihistamines, the active ingredients in most over-the-counter allergy-control drugs - Interfere with histamine activity by binding to histamine receptors on target organs -Have side effects, however, such as drowsiness -New antihistamines lack this side effect because they do not cross the blood-brain barrier -Injectable Epinephrine

16.5 Type IV Hypersensitivities: Cell-Mediated (Delayed) Reactions B. Contact dermatitis

-Caused by exposure to resins in poison ivy or poison oak, to simple haptens in household items like jewelry -The allergen first penetrates the outer skin layers, is process by skin dendritic cells and is presented to T cells -When subsequent exposures attract lymphocytes and macrophages to this area, these cells give off enzymes and inflammatory cytokines that severely damage the epidermis in the immediate vicinity

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms 5. Prostaglandins

-Group of powerful inflammatory agens -Regulate smooth muscle contraction -In allergic reactions, they are responsible for vasodilation, increased vascular permeability, increased senesitivity to pain, and bronchoconstriction. -Nonsteroidal anti-inflammatory drugs, such as aspirin and ibuprofen, work by preventing the actions of prostaglandins

16.1 The Immune Response: A Two-Sided Coin A. Immunopathy 1. Types of Immunopathology

-Immunopathology is the study of disease states associated with overreactivity or underreactivity of the immune response. -Overactivity takes the forms of: 1. Allergies 2. Hypersensitivity 3. Autoimmunity -Hyposensitivity diseases included: 1. Immunodeficiency 2. Immune function is incompletely developed, suppressed, or destroyed

16.5 Type IV Hypersensitivities: Cell-Mediated (Delayed) Reactions C. T cells and their role in organ transplantation 1. The Genetic and Biochemical Basis for Graft Rejection

-MHC markers set the events of graft rejection in motion -The cells of each person can exhibit variability in the pattern of these cell surface molecules, the pattern is identical in different cells of the same person -Similarity is seen in close relatives -When donor tissue displays surface molecules of a different MGC class, the T cells of the recipient (host) will recognize its foreignness and react against it

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis E. IgE and mast cell-mediated allergic conditions 2.Food Allergy

-Mode of entry is intestinal, food allergies can also affect the skin and respiratory tract -Gastrointestinal symptoms include vomiting, diarrhea, and abdominal pain -Other manifestation of food allergies include eczema, hives, rhinitis, asthma, and, occasionally, anaphylaxis

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis E. IgE and mast cell-mediated allergic conditions 3. Drug Allergy

-Reactions range from mild rash to fatal anaphylaxis -Compounds implicated most often are antibiotics (penicillin for Ex.), synthetic antimicrobials(sulfa drugs), aspirin, opiates, and contrast dye used in X rays

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms 6. Bradykinin

-Related to kinins that participate in blood clotting and chemotaxis -In allergic reactions, it causes prolonged smooth muscle contraction of the bronchioles, dilatation of peripheral arterioles, increased capillary permeability, and increased mucus secretion

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms 2. Serotonin

-Role is uncertain in humans but complement histamine -In animals, it increases vascular permeability, capillary dilation, smooth muscle contraction, intestinal peristalsis, and respiratory rate; but it diminishes central nervous system activity

16.5 Type IV Hypersensitivities: Cell-Mediated (Delayed) Reactions C. T cells and their role in organ transplantation 2. T-cell-mediated recognition of foreign MHC receptors b. Graft rejection of host i. Graft versus host disease

-Some grafted tissue (especially bone marrow) contain an indigenous population called passenger lymphocytes -Makes it quite possible for the graft to reject the host, causing Graft Versus Host Disease (GVHD) -Because any host tissue bearing MHC markers foreign to the graft can be attacked, the effects of GVHD are widely systemic and toxic

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms 1. Histamine

-The most profuse and fast-acting allergic mediator -Histamine's actions on smooth muscle vary with location -It Constricts the smooth muscle layers, thereby causing labored breathing and increased intestinal motility -In contrast, histamine relaxes vascular smooth muscle and dilates arterioles and venules. -Is responsible for the Wheal-and-Flare reaction in the skin, pruritus(itching), and headache

16.6 An Inappropriate Response Against Self: Autoimmunity A. Genetic and gender correlation in autoimmune disease

-The precipitating cause of autoimmune disease remains obscure -Susceptibility is determined by genetics and influenced by gender -Cases cluster in families, and even unaffected members tend to develop the autoantibodies for that disease - Women account for nearly 75% of all cases of diagnosed autoimmune disease, but the biological basis for this fact largely remains a mystery -A number of autoimmunities have been linked to genes on the X chromosome, and one hypothesis centers on the role of X-chromosome inactivation in the development of these diseases

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms

-The principal chemical mediators produced by mast cells and basophils are histamine, serotonin, leukotriene, platelet-activating factor, prostalandins, and bradykinin. -These chemicals, acting alone or in combination, account for the tremendous scope of allergic symptoms -Targets of these mediators include the skin, upper respiratory tract, gastrointestinal tract, and conjunctiva -General responses of these organs include rashes, itching, redness, rhinitis, sneezing, diarrhea, and shedding of tears. -Systemic targets include smooth muscle, mucous glands, and nervous tissue -Changes in its activity can profoudly alter blood flow, blood pressure, and respiration -Pain, anxiety, agitation, and lethargy are also attributable to the effects of mediators on the nervous system

16.3 Type II Hypersensitivities: Reactions That Lyse Foreign Cells A. Human Blood Types

-The serum of one person could clump the red blood cells of another -Four distinct types: Type A,B, and O blood groups 1. Type AA or AO genotype has type A blood 2. Type BB or Bo genotypes has Type B blood 3. Genotype AB produces type AB 4. Genotype OO produces Type O -They are named for the dominant antigen -The RBCs of type O persons have antigens but not A and B antigens -Tissues other than RBCs carry A and B antigens

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System A. Primary Immunodeficiency Diseases 3. Severe combined immunodeficiencies: Dysfunction in B and T cells -Forms

-Two most common forms, Swiss-type agammaglobulinemia and thymic alymphoplasia, genetic defects in development of the lymphoid cell line result in extremely low numbers of all lymphocyte types and poorly developed humoral and cellular immunity -Adenosine deaminase (ADA) deficiency, is caused by an autosomal recessive defect in the metabolism of adenosine -Lymphocytes develop but a metabolic product builds up abnormally and selectively destroys them

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis D. Cytokines, Target Organs, and Allergic Symptoms 3. Leukotriene

-Was known as the "slow-reacting substance of anaphylaxis" for its property of inducing gradual contraction of smooth muscle -Responsible for the prolonged bronchospasm, vascular permeability, and mucus secretion of the asthmatic individual

16.5 Type IV Hypersensitivities: Cell-Mediated (Delayed) Reactions C. T cells and their role in organ transplantation 2. T-cell-mediated recognition of foreign MHC receptors a. Host rejection of Graft

-When the cytotoxic T cells of a host recognize foreign class I MHC markers on the surface of grafted cells, they release interleukin-2 as part of a general immune mobilization -Antigen-specific helper and cytotoxic T cells bind to the grafted tissue and secrete lymphokines that begin the rejection process within 2 weeks of transplantation -Late in this process, antibodies formed against the graft tissue contribute to immune damage resulting in the destruction of the vascular supply and death of the graft

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis B. The nature of allergens and their portals of entry (6)

1. Chemical Nature - Proteins are more allergenic than carbohydrates, fats, or nucleic acids 2. Haptens & Allergens - Some allergens are haptens, nonproteinaceous substances with a molecular weight of less than 1,000, which can form complexes with carrier molecules in the body 3. Inhalants - Airborne environmental allergens such as: Pollen, Dust, Mold Spores, Dander, Animal Hair, Insect Parts, Formalin, Drugs 4. Ingestant - Allergens that enter by mouth such as: Food (milk, peanuts, wheat, shellfish, nuts, eggs, fruits), Food Additivies, Drugs (penicillin) 5. Injectant - Allergies are triggered by: Bee or Wasp Venom, Drugs, Vaccines, Serum, Enzymes 6. Contactant - Allergens that enter through the skin

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis F. Anaphylaxis: An overpowering systemic reaction

1. Cutaneous -Anaphylaxis, or anaphylactic shock, was first used to denote a reaction of animals injected with a foreign protein -Animals showed no response during the first contact, upon reinoculation with same protein at a later time, they exhibited acute symptoms--itching, sneezing, difficult breathing, prostration, and convulsions--and many died in a few minutes. 2. Systemic and shock -Systemic anaphylaxis is characterized by sudden respiratory and circulatory disruption that can be fatal in a few minutes. -In humans, the allergen and route of entry are variable, though bee stings and injections of antibiotics or serum are implicated most often -Can create a sensitivity that can last for decades after exposure

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis G. Diagnosis of Allergy 1. Histamine Release Test

1. Histamine Release Test -Allergies mimic infections and other conditions, it is important to determine if a person is actually allergic and to identify the specific allergen or allergens involved -Allergy diagnosis involves several levels of tests, including nonspecific, specific, in vitro, and in vivo methods. -Radioallergosorbent test (RAST), which measures levels of IgE to spefic allergens -Measures levels of IgE to specific allergens -Cannot be used for all allergens, but recently more tests have become available for food allergens -Specific in vitro tests can determine the allergic potential of a patient's blood sample

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System A. Primary Immunodeficiency Diseases

1. Primary Diseases - Present at birth and usually stemming from genetic erros 2. Secondary Diseases - Acquired after birth and caused by natural or artificial agens

16.4 Type III Hypersensitivities: Immune Complex Reactions B. Types of Immune Complex Diesease

1. The Arthus Reaction --A localized dermal injury due to inflamed blood vessels in the vicinity of any injected antigen -Usually an acute response to a second injection of vaccines or drugs at the same sit as the first injection -Area becomes red, hot to the touch, swollen, and very painful -Due to destruction of tissues and release of histamines 2. Serum Sickness -A systemic injury initiated by antigen-antibody complexes that circulate in the blood and settle into membranes at various sites -Caused by injections of animal hormones and drugs -The immune complexes enter the circulation; are carried throughout the body; and are eventually deposited in blood vessels of the kidney, heart, skin, and joints

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis C. Mechanisms of Type I allergy: Sensitization and Provocation 2. The role of mast cells and basophils

1. Their ubiquitous location in tissues. Mast cells are located in the connective tissue of virtually all organs 2. Their capacity to bind IgE during sensitization and degranulate. Each cell carries 30,000 to 100,000 cell receptors, which trigger the release of inflammatory cytokines from cytoplasmic granules when bound by allergen-associated IgE

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis G. Diagnosis of Allergy 2. Skin Testing (in vivo)

2. Skin Testing (in vivo) -in vivo method to detect precise atopic or anaphylactic sensitivities is skin testing -A patient's skin is injected, scratched, or pricked with a small amount of a pure allergen extract -Hundreds of these allergen extracts containing common airborne allergens and more unusual allergens -Unfortunately, skin tests for food allergies using food extracts are unreliable in most cases -The allergist maps the skin on the inner aspect of the forearms or back and injects the allergens intradermally according to this predetermined pattern -20 minutes after antigenic challenge, each site is appraised for a wheal response indicative of histamine release

16.3 Type II Hypersensitivities: Reactions That Lyse Foreign Cells C. Antibodies against A and B antigens 1. Clinical Concerns in Transfusions

A. Universal Donor and Universal Recipient - First, the individual blood types of donor and recipient must be determined -By used of a standard technique, drops of blood are mixed with antisera that contain antibodies against the A and B antigens and are then observed for the evidence of agglutination -Universal Donor - Type O blood, lacks A and B antigens and will not be agglutinated by other blood types -Universal Recipient - type AB blood, lacks agglutinating antibodies, an individual with this blood could conceivably receive any type of blood B. Transfusions Reactions - Transfusion of the wrong blood type causes differing degrees of adverse reaction -The most severe reaction is massive hemolysis when the donated red blood cells react with recipient antibody and trigger the complement cascade -The resultant destruction of red cells leads to systemic shock and kidney failure brought on by the blockage of glomeruli by cell debris -Death is a common outcome

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System B. Secondary Immunodeficiency Diseases 1. Causes a. HIV/AIDs

AIDs -Caused when several types of immune cells, including T helper cells, monocytes, macrophages, and antigen-presenting cells, are infected by the human immunodeficiency virus (HIV) -The depletion of T helper cells and functional impairment of immune responses ultimately account for the cancers and opportunistic protozoan, fungal, and viral infections associated with this disease

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis C. Mechanisms of Type I allergy: Sensitization and Provocation 1. The physiology of IgE-medicated allergies

After contact with allergen... -Plasma cells produce immunoglobulin E (IgE), the antibody of allergy -IgE is different from other immunoglobulins in having a Fc region with great affinity for mast cells and basophils -The binding IgE to these cells in the tissues sets the scene for the reactions that occur upon repeated exposure to the same allergen

16.6 An Inappropriate Response Against Self: Autoimmunity B. The Origins of Autoimmune disease 3. Bystander Effect

Bystander Effect -T-cell activation may incorrectly "turn on" B cells that can react with self antigens, or chemical compounds such as heavy metals may stimulate autoreactive T-cell populations within the immune system

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System B. Secondary Immunodeficiency Diseases 1. Causes b. Cancers

Cancers -Cancers that target the bone marrow or lymphoid organs can be responsible for extreme malfunction of both humoral and cellular immunity -Leukemia, a massive number of cancer cells compete for space and literally displace the normal cells of the bone marrow and blood -Plasma cell tumors produce large amounts of nonfunctional antibodies, and thymus tumors cause severe T-cell deficiencies

16.6 An Inappropriate Response Against Self: Autoimmunity B. The Origins of Autoimmune disease 2. Clonal Selection Theory

Clonal Selection Theory -Forbidden Clones erroneously target self tissues leading to autoimmunity -The immune system of a fetus develops tolerance by eradicating all self-reacting lymphocyte clones, called forbidden clons -Some of these forbidden clones may survive -These autoreactive B or T cells can inappropriately attack tissues with self antigens

16.6 An Inappropriate Response Against Self: Autoimmunity C. Examples of Autoimmune Disease 2. Autoimmunities of the endocrine glands

Graves' Disease -Cause of Graves' disease is the attachment of autoantibodies to receptors on the thyroxin-secreting follicle cells of the thyroid gland -Causes the overproduction of this hormone and the symptoms of hyperthyroidism, which affect nearly every body system Type I Diabetes -Insulin, secreted by the beta cells in the pancreas, regulates and is essential to the utilization of glucose by cells -Molecular mimicry has been implicated in the sensitization of cytoxic cells in type I diabetes -Leads to lysis of pancreatic cells

16.6 An Inappropriate Response Against Self: Autoimmunity B. The Origins of Autoimmune disease 4. Molecular Mimicry

Molecular Mimicry -Leads the immune system to misidentify self antigens -Microbial antigens display molecular determinants similar to normal human cells -An infection could cause formation of antibodies that can cross-react with tissues -Mimicry of bacterial antigens has led to rheumatic fever and psoriasis -Type I diabetes and multiple sclerosis are disorders possibly triggered by viral infection

16.6 An Inappropriate Response Against Self: Autoimmunity C. Examples of Autoimmune Disease 3. Neuromuscular Autoimmunities

Myasthenia -Myasthenia gravis is a syndrome caused by autoantibodies binding to the receptors for acetylcholine, a chemical required to transmit a nerve impulse across the synaptic junction to a muscle -Severely damages the muscle cell membrane that transmission is blocked and paralysis ensues -Leads to complete loss of skeletal muscle function Multiple Sclerosis -A paralyzing neuromuscular disease associated with lesions in the insulating myelin sheath that surrounds neurons of the central nervous system -T-cell induced and autoantibody-induced damage severely compromises the capacity of neurons to send impulses, resulting in muscular weakness and tremors, difficulties in speech and vision, and some degree of paralysis

16.6 An Inappropriate Response Against Self: Autoimmunity C. Examples of Autoimmune Disease 1. Systemic Autoimmunities

SLE -Systemic lupus erythematosus, or lupus, is named so because of the characteristic butterfly-shaped rash that drapes across the nose and cheeks -All SLE patients produce autoantibodies against a great variety of organs and tissues or intracellular materials, such as nucleoprotein of the nucleus and mitochondria -Kidney failure, blood abnormalities, lung inflammation, myocarditis, and skin lesions are the predominant symptoms Rheumatoid Arthritis -Incurs progressive, debilitating damage to the joints -In some patients, the lugs, eyes, skin, and nervous system are also involved -In the joint form of the disease, autoantibodies form immune complexes that bind to the synovial membrane of the joints and activate phagocytes and stimulate release of cytokines

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System B. Secondary Immunodeficiency Diseases 1. Causes

Secondary acquired deficiencies in B cells and T cells are caused by one of four general agents: 1. Infection 2. Noninfectious metabolic disease 3. Chemotherapy 4. Radiation

16.6 An Inappropriate Response Against Self: Autoimmunity B. The Origins of Autoimmune disease 1. Sequestered Antigens

Sequestered Antigens During embryonic growth, some tissues are immunologically privileged -Some of these antigens become exposed by means of infection, trauma, or deterioration -When they finally encounter immune cells, they are recognized as a foreign substance, triggering areaction to self antigen

16.7 Immunodeficiency Diseases: Hyposensitivity of the Immune System A. Primary Immunodeficiency Diseases 3. Severe combined immunodeficiencies: Dysfunction in B and T cells

Severe Combined Immunodeficiencies (SCIDs) -Most serious and potentially lethal forms of immunodeficiency because they involve dysfunction in both lymphocyte systems -Are due to the complete absence of the lymphocyte stem cell in the marrow; others are attributable to the dysfunction of B cells and T cells later in development -Infants with SCIDs usually manifest the T-cell deficiencies within days after birth

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis H. Treatment and prevention of allergy 1. Three rationales

a. Avoiding the allergen, although this may be very difficult in many instances b. Taking drugs that block the action of lymphocytes, mast cells, or chemical mediators; and c. Using injections of the allergen in such a way that the allergic reaction is short-circuited

16.2 Type I Allergic Reactions: Atopy and Anaphylaxis E. IgE and mast cell-mediated allergic conditions 1. Atopic Diseases

a. Hay fever is a generic term for allergic rhinitis, a seasonal reaction to inhaled plant pollen or molds, or a chronic, year-round reaction to a wide spectrum of airborne allergens or inhalants b. Asthma is a respiratory disease characterized by episodes of impaired breathing due to severe bronchoconstriction. The airways of asthmatic people are exquisitely responsive to minute amounts of inhalant allergens, food, or other stimuli, such as infectious agents c. Eczema, or atopic dermatitis is an intensely itchy inflammatory condition of the skin. Sensitization occurs through ingestion, inhalation, and, occasionally, skin contact with allergens. Begins in infancy with reddened, vesicular, weeping, encrusted skin lesions Progresses in childhood and adulthood to a dry, scaly, thickened skin condition

16.5 Type IV Hypersensitivities: Cell-Mediated (Delayed) Reactions C. T cells and their role in organ transplantation 2. T-cell-mediated recognition of foreign MHC receptors c. Classes of grafts

i. Autograft - Tissue transplanted from one site on an individual's body to another site on his or her body ii. Isograft - Tissue from an identical twin is used iii. Alllografts - The most common type of grafts, are exchanges between genetically different individuals belonging to the same species. A close genetic correlation is sought for most of these grafts iv. Xenograft - A tissue exchange between individuals of different species