Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
Immunoglobulins, or antibodies, are the chemicals released by activated B cells. (They also act as surface proteins in B cells.) Their role is to attach to invaders or foreign substances - either this will in itself disrupt the invader's potential to cause trouble, or the attached antibodies can serve as a signal to macrophages and natural killer cells that here is something to be destroyed.
Immunoglobulins generally have a structure as shown in the following diagram. They are made up of four protein chains - two longer ones (the "heavy chains") and two shorter ones (the "light chains") - connected together with disulphide bonds into a Y-shaped structure.
Each of the protein chain in an immunoglobulin folds up into bulges called domains. A heavy chain has four domains, and a light chain only two.
The domains at the "top" of the Y-shape are variable - the genes which encode them are done in such a way that the protein sequence could be one of literally thousands of possibilities. This enables antibody to be produced which recognises millions of different types of substance, since recognition is carried out by fitting the substance into one of the two tips of the Y-shape.
Immunoglobulins are related to other molecules which you'll find in this hypertext. As the following diagram illustrates, the "immunoglobulin superfamily" contains CD4, CD8, the T Cell Receptor (TCR), and HLA.
Immunoglobulins are often grouped into subclasses called IgG, IgD, IgE, IgA, IgM. These arise simply because there are five different sorts of heavy chain. The configuration of the heavy chains in IgM, for example, is such that IgM forms into pentamers (five molecules joined together), and forms a five-pointed star shape. IgA, however, ordinarily exists as dimers (i.e. two molecules together).