System used to describe types of allergic (hypersensitivity) reactions. Type 1, anaphylactic typeÑIgE binds mast cells and basophils via Fc fragment; type 2, cytotoxic typeÑantigens on cell surface bind antibodies causing opsonization, phagocytosis and variably complement binding and activation; type 3, complex-mediated typeÑantigen binds antibody to form complexes that activate Hageman factor and complement pathway and cause platelet aggregation; type 4, cell-mediated (delayed) typeÑantigen activates T cells, which proliferate and turn on macrophages and lymphocytes.