General Features of Antibody Structure

Plasma or serum proteins are traditionally separated by solubility characteristics into albumins and globulins and may be more extensively separated by migration in an electric field, a process called electrophoresis. Most antibodies are found in the third fastest migrating group of globulins, named gamma globulins for the third letter of the Greek alphabet. Another common name for antibody is immunoglobulin (Ig), referring to the immunity-conferring portion of the gamma globulin fraction. The terms immunoglobulin and antibody are used interchangeably throughout this book.

All antibody molecules share the same basic structural characteristics but display remarkable variability in the regions that bind antigens. This variability of the antigen-binding regions accounts for the capacity of different antibodies to bind a tremendous number of structurally diverse antigens. There are believed to be a million or more different antibody molecules in every individual (theoretically, the antibody repertoire may include more than 1011 different antibodies), each with unique amino acid sequences in their antigen-combining sites. The effector functions and common physicochemi-cal properties of antibodies are associated with the non-antigen-binding portions, which exhibit relatively few variations among different antibodies.

An antibody molecule has a symmetric core structure composed of two identical light chains and two identical heavy chains (Fig. 5-1). Both the light chains and the heavy chains contain a series of repeating, homologous units, each about 110 amino acid residues in length, that fold independently in a globular motif that is called an Ig domain. An Ig domain contains two layers of P-pleated sheet, each layer composed of three to five strands of antiparallel polypeptide chain (Fig. 5-2). The two layers are held together by a disulfide bridge, and adjacent strands of each P sheet are connected by short loops. It is the amino acids in some of these loops that are the most variable and critical for antigen recognition, as discussed later.

Both heavy chains and light chains consist of amino-terminal variable (V) regions that participate in antigen recognition and carboxyl-terminal constant (C) regions; the C regions of the heavy chains mediate effector functions. In the heavy chains, the V region is composed of

Secreted IgG

Heavy chain

Antigen-binding site

Heavy chain

Antigen-binding site

Fc receptor/ complement ■ binding sites

Light chain

Tail piece

B Membrane IgM

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