Structure and function of antibodies

Soluble antibodies are Y-shaped glycoproteins of the immunoglobulin (Ig) family synthesized and secreted by plasma cells in response to antigenic stimulation of naive B cells. Occurring freely in the blood and lymph, antibodies act as flexible adapters, allowing various mediators of the immune system to recognize specific pathogens and their products and to exert their action. The body can make several million different antibodies, each one able to recognize a specific infectious agent.

All antibody molecules consist of four polypeptide chains - two identical heavy chains and two identical light chains. Each of the arms of the Y-shaped molecule represents a Fab region, which binds to antigen; the stem represents the Fc region, which interacts with immune system mediators. The terminal half of each of the Y arms is highly variable in its amino acid sequence from one antibody to the next, the remainder of the molecule being relatively constant. These differences give rise to the terms 'variable' and 'constant' regions. Immunoglobulins are classified on the basis of structural differences in the heavy chains. The most important classes are IgG, IgA, IgM, IgD and IgE. The constant regions of the heavy chains, and consequently the class of the antibody, can change during the course of an immune response, although the antigen specificity of the antibody is unchanged. This process is known as antibody class switching and as a result the effector functions performed by the antibody will also change.

Phagocytes and natural killer cells, among other cells, have Fc receptors on their surfaces, allowing these cells to bind and destroy antibody-coated bacteria and tumour cells. Antibodies can also bind to the first component (Clq) of the complement system, thereby activating the complement cascade.

The primary function of an antibody is to bind antigens. In a few cases this has a direct effect; for example, neutralization of toxins by blocking active sites and inhibition of viral attachment to the target cell or penetration of the cell. In other cases, including precipitation of soluble antigen, agglutination of particulate antigen and activation of the complement system, secondary effector functions come into play. These secondary functions result from the formation of immune complexes between multivalent antigens and divalent antibody.

The variable region of a particular antibody may represent a unique structure that has appeared for the first time within a certain individual. Consequently, an immune response may be directed against epitopes on the variable domains. Each of these epitopes is known as an idiotope and the sum of all the idiotopes determines the antibody's idiotype. An immune response that is elicited by the host's own antibody molecules is called an auto-anti-idiotype response and the 'anti-antibodies' that are produced are termed auto-anti-idiotype antibodies. It has been postulated (Jerne, 1984) that idiotype and anti-idiotype antibodies can interact to form a network that can influence the outcome of an immune response.

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