As mentioned previously, each B lymphocyte has receptors on its surface for one specific antigen. The molecular structure of the receptor is a globulin-type structure with "domains" of beta-pleated sheets. When the B lymph fully matures, it will exit the bone marrow and migrate to the lymph nodes. Antigen material is brought into the lymph nodes via the lymph flow; if a given B lymph recognizes "its antigen," it will respond by becoming activated, cloning itself, and eventually, the daughter cells (now called Plasma Cells) will synthesize and secrete antibodies or immunoglobulins, which are nearly the same structure as that of the original receptor.
The image to the top right shows a simplified version of the structure. Note the areas labeled antigen-binding sites. These antigen-binding sites are on the domain colored purple; this domain is known as the "variable domain" because this part of the molecule varies from B lymph to B lymph, and most importantly, it is what confers that specificity to that molecule. The remaining domains colored pink are known as the constant domains, and they are structurally the same among antibody classes. There are five classes of antibodies, each of which has a somewhat different type of activity. The classes are named after the designations of the "heavy chains" (the longer chains). The five different heavy chains are alpha, gamma, delta, epsilon, and mu - thus, the classes are known as IgA, IgG, IgD, IgE, and IgM.
So, for instance, all IgM molecules on all B cells have approximately the same constant structure, but each varies at the variable domain.
The second image (bottom right) shows a slightly more detailed version of one class (IgM) of an immunoglobulin or antibody molecule. Note the domains that are held together with a disulfide bond (s-s). It is shown along with many other receptors with different immune functions. Note that this domain-type structure is very common among many immune receptors, not just immunoglobulin receptors, and thus is referred to as the "immunoglobulin superfamily." All domains marked "c" are constant among their class, and domains marked "v" vary from cell to cell.
