Final answer:
Type II hypersensitivity reactions are characterized by antibody-mediated responses against cellular or extracellular matrix antigens, involving IgG and IgM antibodies that can activate complement or elicit ADCC.
Step-by-step explanation:
Type II hypersensitivity reactions are characterized by antibody-mediated responses against cellular or extracellular matrix antigens. These reactions involve the binding of IgG and IgM antibodies to cell-surface antigens or to antigens associated with the extracellular matrix, like basement membranes. Following the antibody binding, two primary mechanisms may occur: one involves the activation of the complement system, leading to an inflammatory response and cell lysis, and the other involves antibody-dependent cell-mediated cytotoxicity (ADCC), with cytotoxic T cells engaging in the destruction of the targeted cells.
These mechanisms are distinctly different from the actions of IgE in type I hypersensitivities, which cause mast cell degranulation, and from immune complex-mediated type III reactions. Moreover, type II reactions differ from type IV, which are T-cell-mediated responses.