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112         SECTIon II    Immunology  ` Immunology—Immune reSponSeS                                                                                             Immunology  ` Immunology—Immune reSponSeS





               Hypersensitivity types Four types (ABCD): Anaphylactic and Atopic (type I), AntiBody-mediated (type II), Immune
                                      Complex (type III), Delayed (cell-mediated, type IV). Types I, II, and III are all antibody-mediated.
                Type I               Anaphylactic and atopic—two phases:       First (type) and Fast (anaphylaxis).
                 hypersensitivity        ƒ Immediate (minutes): antigen crosslinks   Test: skin test or blood test (ELISA) for allergen-
                                        preformed IgE on presensitized mast cells   specific IgE.
               Allergen    Allergen-
                           specific IgE  Ž immediate degranulation Ž release of   Example:
                           Fc receptor  histamine (a vasoactive amine) and tryptase     ƒ Anaphylaxis (eg, food, drug, or bee sting
                           for IgE
                                        (a marker of mast cell activation).       allergies)
                                         ƒ Late (hours): chemokines (attract       ƒ Allergic asthma
                                        inflammatory cells, eg, eosinophils) and
                                        other mediators (eg, leukotrienes) from mast
                                        cells Ž inflammation and tissue damage.

                   Degranulation
                Type II              Antibodies bind to cell-surface antigens   Direct Coombs test—detects antibodies
                 hypersensitivity     Ž cellular destruction, inflammation, and   attached directly to the RBC surface.
                                      cellular dysfunction.                    Indirect Coombs test—detects presence of
                          NK cell
                                                                                unbound antibodies in the serum
                                     Cellular destruction—cell is opsonized (coated)   Examples:
                                      by antibodies, leading to either:            ƒ Autoimmune-hemolytic anemia
                                         ƒ Phagocytosis and/or activation of       ƒ Immune thrombocytopenia
                                        complement system.                         ƒ Transfusion reactions
               Fc receptor               ƒ NK cell killing (antibody-dependent cellular     ƒ Hemolytic disease of the newborn
                 for IgG
                                        cytotoxicity).
               Surface antigen
                                     Inflammation—binding of antibodies to cell   Examples:
                  Abnormal cell       surfaces Ž activation of complement system     ƒ Goodpasture syndrome
               Antibody-dependent     and Fc receptor-mediated inflammation.       ƒ Rheumatic fever
               cellular cytotoxicity
                                                                                   ƒ Hyperacute transplant rejection
                                     Cellular dysfunction—antibodies bind to cell   Examples:
                                      surface receptors Ž abnormal blockade or     ƒ Myasthenia gravis
                                      activation of downstream process.            ƒ Graves disease
                                                                                   ƒ Pemphigus vulgaris







































          FAS1_2019_02-Immunology.indd   112                                                                            11/7/19   3:24 PM
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