15.1: Antibodies are produced in response to antigens

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Antibodies are proteins produced by vertebrates with adaptive immune systems capable of responding to foreign antigens. Antigens are defined as substances that stimulate the production of antibodies. Antigens are commonly able to stimulate the production of multiple kinds of antibodies, each of which recognizes a small, distinct region on the surface of the antigen known as an epitope. Antibodies are Y-shaped proteins produced by lymphocytes that bind epitopes with high affinity.

Antibodies binding to an antigen.

An antigen with three different epitopes on its surface is bound by three different antibody molecules, each of which binds a singleepitope with high affinity.

The availability of hybridoma cells that secrete large quantities of antibodies with a single specificity has greatly facilitated structural studies on antibodies. Researchers are able to harvest antibody molecules secreted by cultured hybridoma cells and to prepare crystals for X-ray diffraction. Based on a large number of crystallographic studies, we now understand the basic architecture of antibodies, more properly known as immunoglobins. The crystal structures show that immmunoglobins (Igs) are composed of three domains that are readily apparent in the crystal structure (below). The two F_abregions (antigen-binding fragments) that form the arms of the “Y” are hypervariable regions involved in binding antigen. The F_c region (crystallizable fragment) that forms the base of the “Y” is recognized by non-immune effector cells, such as mast cells and macrophages, which process antigen-antibody complexes. Each Ig class has a characteristic heavy chain, which gives the class its name. We are using antibodies from the IgG class of immunoglobins, which have gamma heavy chains. (IgGs are also known as gamma globulins.) IgA molecules have alpha chains, IgM molecules have mu chains, etc.

Crystal structure of an IgG antibody.

This figure is derived from Protein Data Bankentry 1IGT (Harris et al., 1997).