We now know that these neutralizing substances are antibody molecules and, in the serum of vertebrates, there are millions of distinct antibody molecules present. The constant regions are localized toward the COOH terminal region of the molecule, while the variable regions are located at the NH 2 terminus. The variable region is the critical site for antigen binding and is the distinctive feature that is unique to each individual antibody molecule. In this theory, each antibody producing cell expresses but one antibody receptor, but within an individual, millions of different antibody forming cells exist. When an antigen engages those lymphocytes that have a receptor that can bind the antigen, those lymphocytes are activated, divide and differentiate into a plasma cell.
Plasma cells are specialized cells that produce high levels of serum antibody.
As a result, with time, there is an increase in the level of antibodies in the serum capable of binding to the antigen. One important issue that need to be addressed was how an individual generates the millions of different antibodies capable of binding the universe of foreign antigens. J genes all of which can recombine to form a functional heavy chain gene. Since these genes randomly recombine, one can generate over 10,000 unique combinations. Light chain are similar but use only V and J-genes.