(This material is included here for reinforcement. It is also covered in the Immunol-ogy Lecture Notes.)
Type I (immediate) hypersensitivity reactions(anaphylactic type)are character-ized by IgE-dependent release of chemical mediators from mast cells and basophils. Cross-linking of IgE bound to antigen to IgE Fc receptors on the surface of mast cells and basophils causes degranulation. This binding triggers release of chemi-cal mediators that include histamine and heparin; eosinophil chemotactic factor; leukotriene B4 and neutrophil chemotactic factor; and prostaglandin D4, platelet-activating factor (PAF), and leukotrienes C4 and D4. Influx of eosinophils amplifies and perpetuates the reaction. Effects may be systemic (anaphylaxis, as for example due to bee stings or drugs) or localized (food allergies, atopy, and asthma).
Type II hypersensitivity reactions (antibody-mediated) are mediated by IgG or IgMantibodies directed against a specific target cell or tissue. Reactions can take several forms.
· In complement-dependent cytotoxicity, fixation of complement results in osmotic lysis or opsonization of antibody-coated cells; examples include auto-immune hemolytic anemia, transfusion reactions, and erythroblastosis fetalis.
· In antibody-dependent cell-mediated cytoxicity (ADCC), cytotoxic killing of an antibody-coated cell occurs; an example is pernicious anemia. Antireceptor antibodies can activate or interfere with receptors; examples include Graves disease and myasthenia gravis.
Type III hypersensitivity reactions(immune complex disease)are characterized bythe formation of in situ or circulating antibody-antigen immune complexes, which deposit in tissue resulting in inflammation and tissue injury. Examples include serum sickness, systemic lupus erythematosus (SLE), and glomerulonephritis.
Type IV hypersensitivity reactions(cell-mediated type)are mediated by sensitizedT lymphocytes. In delayed type hypersensitivity, CD4+ TH1 lymphocytes mediate granuloma formation; examples include the PPD skin test and tuberculosis.
In cytotoxic T-cell–mediated hypersensitivity, CD8+ T-cell lymphocytes destroy antigen- containing cells; examples include type 1 diabetes, virus-infected cells, immune reaction to tumor-associated antigens, and graft rejection.