Antibodies- Produced by plasma cells; known collectively as immunoglobulins (Ig) and consist of two identical heavy chains and light chains linked by disulfide bonds
- Each Ab with unique domain which recognizes and binds Ag epitope
- Ab function: neutralize microbes/toxins by direct binding, through enhanced opsonization and/or lysis (latter via complement activation)
- Different classes based on structure of heavy chain; Ig class switching regulated by cytokines derived from T cells
| | | | | | Fc stands for fragment that spontaneous crystallizes | | | | | |
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- Neutralizing Abs: IgA1/IgA2 (mucosal), IgG2/IgG4 (tissue)
- Cleavage of immunoglobulin by papain results in two identical Fab fragments (antigen-binding fragment) and one Fc fragment (involved in complement activation and opsonization)
| | | | | | | Table 1-16 Classes of Immunoglobulins | | | Ig | | Characteristics | | | IgA | | Dimer, predominantly in mucosal surfaces, can activate complement system via alternative pathway (not classical pathway) | | | IgD | | Little known function; present as surface receptor on mature B cells | | | IgE | | Classic anaphylactoid antibody; binds allergens and releases mast cell mediators; increased in atopic patients | | | IgG | | Most abundant Ig; four subclasses (IgG1–IgG4) which differ in ability to activate complement, IgG1/IgG3 (potent activators) > IgG2 > IgG4 (weak to none), able to cross placenta, abundant production with secondary response, best for opsonizing/fixing complement | | | IgM | | Pentamer (five Ig molecules), largest in size, major Ig production in primary immune response, most efficient at activating complement cascade, not involved in opsonization, does not cross placenta | | | | | |
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