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  1. Vaccines and Immunizations
    1. Active immunization-the protection of humans and domestic animals from communicable disease by the administration of vaccines
      1. Vaccine-a preparation containing one or more antigens of a pathogen; vaccines and vaccination have a long history starting with Jenner's use of cowpox as a vaccine against smallpox; today there are many vaccines and vaccination is one of the most cost-effective methods for preventing microbial disease
      2. Immunization practices depend on the age of the individual and the risk group to which the individual belongs
        1. Children begin a vaccination series at 2 months of age; the series protects against numerous childhood disease (e.g., measles, mumps, rubella)
        2. Adults living in close quarters, having reduced immunity, traveling in other countries, and working in certain professions (e.g., health care provider) may receive additional immunizations
      3. Active immunity differs from passive immunity; in artificial immunity, an individual is injected with preformed antibodies that have been produced in another animal, in another human, and in vitro
    2. Types of vaccines and their characteristics-we have many vaccines against acute illnesses (those that progress rapidly either to resolution or to death); we do not have vaccines against chronic illness; many approaches have been used to develop vaccines and the new area of vaccinomics (application of genomics and bioinformatics to vaccine development) is a promising new approach
      1. Whole organism vaccines
        1. Consist of whole organisms that have been inactivated (killed) or attenuated (live but avirulent); in general, attenuated whole-organism vaccines are most effective and easy to use, and they provide more complete immunity
        2. Though considered the "gold-standard", numerous problems are associated with whole-organism vaccines
          1. ) Fail to shield against some diseases
          2. ) Attenuated vaccines can cause disease in immunocompromised individuals
          3. ) Attenuated viruses can revert to virulence
          4. ) Molecules unimportant to establishing immunity can trigger allergic reactions; contaminants in preparation can also cause allergic reactions
      2. Purified molecules as vaccines-vaccines containing specific, purified macromolecules derived from pathogen (capsular polysaccharides, recombinant surface antigens, and inactivated exotoxins called toxoids); avoid many of common risks associated with whole organism vaccines
      3. Recombinant vector vaccines-vaccines containing genetically engineered viruses or bacteria, having genes that encode major antigens from a pathogen; elicit both humoral and cellular immunity
      4. DNA vaccines-vaccines containing recombinant DNA molecules (usually a plasmid); the DNA is taken up by muscle cells after injection and enters host nuclei; the antigen gene is then expressed, producing antigenic proteins that elicit both humoral and cellular immunity; currently several human trials of DNA vaccines are underway
  2. Immune Disorders
    1. Hypersensitivities-exaggerated or inappropriate immune responses that result in tissue damage to the individual
      1. Type I hypersensitivity-includes allergic reactions
        1. Occurs immediately following second contact with responsible antigen (allergen); on first exposure, B cells form plasma cells that produce IgE (reagin), which binds to mast cells or basophils via Fc receptors and sensitizes them; upon subsequent exposure, the allergen binds to these IgE-bearing cells; physiological mediators released by this binding cause anaphylaxis (smooth muscle contraction, vasodilation, increased vascular permeability, and mucus secretion)
        2. Systemic anaphylaxis results from a massive release of these mediators, which cause respiratory impairment, lowered blood pressure, and serious circulatory shock; death can occur within a few minutes
        3. Localized anaphylaxis (atopic) includes hay fever (upper respiratory tract), bronchial asthma (lower respiratory tract), and hives (food allergy)
        4. Skin testing is used to identify allergens; small amounts of possible allergens are inoculated into skin; rapid inflammatory reaction indicates sensitivity
        5. Desensitization to allergens involves controlled exposure to the allergen in order to stimulate IgG production; IgG molecules serve as blocking antibodies that intercept and neutralize the allergen before it can bind to the IgE-bound mast cells
      2. Type II hypersensitivity-generally cytolytic or cytotoxic reaction that destroys host cells
        1. IgG or IgM antibodies are directed against cell surface or tissue antigens; this stimulates complement pathway and a variety of immune effector cells
        2. An example is a blood transfusion reaction in which donated blood cells are attacked by the recipient's antibodies
      3. Type III hypersensitivity
        1. Involves formation of immune complexes, which in the presence of excess antigen are not efficiently removed; their accumulation triggers complement-mediated inflammation, and this can cause vasculitis (inflammation of the blood vessels), glomerulonephritis (inflammation of the kidney glomerular basement membranes), and arthritis (inflammation of the joints)
        2. Diseases resulting from type III reactions
          1. ) Persistent viral, bacterial, or protozoan infection, combined with a weak antibody response, leads to chronic immune complex formation and deposition in the tissues of the host
          2. ) Prolonged production of autoantibodies and chronic immune complex formation leads to immune complex deposition in the tissues (e.g., systemic lupus erythematosus)
          3. ) Repeated inhalation of allergens can cause immune complex deposition at body surfaces (e.g., in the lungs in farmer's lung disease)
      4. Type IV hypersensitivity-involves TDTH lymphocytes (same as TH1), which migrate to and accumulate near the antigen
        1. Presentation of antigen to TDTH causes the cell to proliferate and release cytokines; these attract macrophages and basophils to the area, leading to inflammatory reactions that can cause extensive tissue damage
        2. Can be used diagnostically, as in the tuberculin skin test
        3. Examples of type IV hypersensitivities include allergic contact dermatitis (poison ivy, cosmetic allergies) and some chronic diseases (leprosy, tuberculosis, leishmaniasis, candidiasis, herpes simplex lesions)
    2. Autoimmune Diseases-autoimmunity is characterized by the presence of autoantibodies and is a natural consequence of aging; autoimmune disease results from activation of self-reactive T and B cells that lead to tissue damage
      1. Viruses, genetic factors, hormones, and psychoneuroimmunological factors all influence the development of autoimmune disease
      2. More common in older individuals; may involve viral or bacterial infections that cause tissue damage and the release of abnormally large quantities of antigen, or that cause some self-proteins to alter their form so that they are no longer recognized as self
    3. Transplantation (Tissue) Rejection
      1. Transplantation of tissue from one individual to another can be an allograft (donor and recipient are genetically different individuals of the same species) or xenograft (donor and recipient are different species)
      2. Mechanisms of tissue rejection
        1. Foreign class II MHC antigens trigger TH cells to help TC cells destroy the graft; the TC cells recognize the graft as foreign by detecting the class I MHC antigens of the graft
        2. TH cells may react directly with the graft, releasing cytokines that stimulate macrophages to enter the graft and destroy it
      3. Graft vs. Host Reaction-immunocompetent cells in donor tissue (e.g., bone marrow) reject the immunosuppressed host
    4. Immunodeficiencies-failure to recognize and/or respond properly to antigens
      1. Primary (congenital) immunodeficiencies result from a genetic disorder
      2. Secondary (acquired) immunodeficiencies result from infection by immunosuppressive microorganisms (e.g., AIDS, chronic mucocutaneous candidiasis)
  3. Antigen-Antibody Interactions In Vitro
    1. Many of the antigen-antibody interactions that occur in vivo also occur in vitro and are frequently the basis of diagnostic procedures; serology is the branch of immunology concerned with these in vitro reactions
    2. Agglutination-visible clumps or aggregates of cells or of coated latex microspheres; if red blood cells are agglutinated, the reaction is called hemagglutination
      1. Widal Test-direct agglutination test for diagnosing typhoid fever
      2. Latex agglutination tests are used in pregnancy test; to diagnose mycotic, helminthic, and bacterial infections; and in drug testing
      3. Viral hemagglutination inhibition tests are used to diagnose influenza and other viral infections
      4. Agglutination tests can be used to measure antibody titer (the reciprocal of the greatest dilution showing agglutination reaction)
    3. Complement fixation-used to detect the presence of serum antibodies to a pathogen; currently used to diagnose certain viral, fungal, rickettsial, chlamydial and protozoan diseases
    4. Enzyme-linked immunosorbent assay (ELISA)-involves linking labeled enzymes to an antibody
      1. Double antibody sandwich assay-detects antigens in a sample
        1. Wells of a microtiter plate are coated with antibody specific to the antigen of interest
        2. Test sample is placed in well; if it contains the antigen of interest, the antigen will be retained in the well after washing
        3. Second antibody is added; it is conjugated to an enzyme and is specific to the antigen; the second antibody will be retained in the well after washing if the antigen was retained in the previous step
        4. Substrate of enzyme is added; reaction only occurs if conjugated enzyme (and therefore antigen) is present in the well; produces a colored product that can be detected
      2. Indirect immunosorbent assay-detects serum antibody
        1. Well of a micro titer plate is coated with antigen specific to the antibody of interest
        2. Test serum is added; if antibodies are present, they will bind antigen and will be retained after washing
        3. An antibody against the test immunoglobulin is added; the second antibody is conjugated to an enzyme and will only be retained in the well after washing if the test antibody is present in the well
        4. Substrate of the enzyme is added; reaction only occurs if conjugated antibody (and therefore test antibody) are present in the well; the colored product of the reaction can be detected spectrophotometrically
    5. Flow cytometry and fluorescence
      1. Detects single or multiple microorganisms on the basis of a cytometric parameter or by means of fluorochromes
      2. Flow cytometer forces cells through a laser beam and measures light scatter or fluorescence as the cells pass through the beam; cells can be tagged with fluorescent antibody directed against specific surface antigen
    6. Immunoblotting (Western Blot)-proteins are separated by electrophoresis, blotted to nitrocellulose sheets, then treated with solution containing enzyme-tagged antibodies
    7. Immunodiffusion-involves the precipitation of immune complexes in an agar gel
      1. Single radial immunodiffusion (RID) assay is quantitative
      2. Double diffusion assay (Ouchterlony technique)-lines of precipitation form where antibodies and antigens have diffused and met; determines whether antigens share identical determinants
    8. Immunoelectrophoresis-antigens are first separated by electrophoresis according to charge, and are then visualized by the precipitation reaction; greater resolution than diffusion assay
    9. Immunofluorescence-dyes coupled to antibody molecules will fluoresce (emit visible light) when irradiated with ultraviolet light
      1. Direct-used to detect antigen-bearing organisms fixed on a microscope slide
      2. Indirect-used to detect the presence of serum antibodies
    10. Immunoprecipitation-soluble antigens form insoluble immune complexes that can be detected
    11. Liposomes-artificially created microscopic lipid vesicles that contain a colored dye in its aqueous compartment and specific antibodies (or antigens) on its surface; will bind to complementary antigens (or antibodies) in a test sample and this is detected by presence of color; used in diagnostic tests for group A streptococci and respiratory syncytial virus
    12. Neutralization-an antibody that is mixed with a toxin or a virus will neutralize the effects of the toxin or the infectivity of the virus; this is determined by subsequent assay in lab animals or tissue culture
    13. Radioimmunoassay (RIA)-purified antigen labeled with a radioisotope competes with unlabeled sample for antibody binding
    14. Serotyping-antigen-antibody specificity is used to differentiate among various strains (serovars) of an organism

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