IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 6. Antibodies and Cell Receptors for Them

EFFECTOR FUNCTIONS OF ANTIBODIES

The primary function of Antibodies is antigen binding. In some cases, this directly achieves a protective effect, such as neutralizing a bacterial toxin or preventing a virus from entering Cells. However, antibody-antigen interactions often remain inconclusive until they execute their secondary, or 'effector,' Functions (see Figs. 6.17 and 6.18).

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Fig. 6.17. The execution of antibody effector functions is mediated by distinct regions within the Fc portion of their molecules. The placental transfer of IgG in humans and its Transport Across the intestinal epithelium in rodents involve an MHC class I-like receptor molecule (see Fig. 6.23).

Fig. 6.18. Ever more members of the heterogeneous family of immunoglobulin-binding receptors are being discovered (only selected Examples are shown here). The FcμR receptor is expressed on activated B cells, but not on T cells or monocytes. In addition to the cells listed in the table, FcεRII is expressed on eosinophils, platelets, T cells, and B cells.

One of the most important effector mechanisms of IgG1 and IgG3 involves the activation of The Complement System—a group of specialized Serum Proteins that participate in inflammatory responses (see Chapter 5). Upon binding to an antigen, IgM, IgG1, and IgG3 can trigger the proteolytic cascade of the complement system. IgG2 is less effective in this regard, whereas IgG4, IgA, IgD, and IgE do not activate complement at all.

Another key effector function of IMMUNOGLOBULINS is their selective interaction with various Cell types via specialized cell-surface receptors (Fig. 6.18).