ABSTRACT
The human genome is thought to contain fewer than 10 genes, yet a human can make at least 10 different types of antibody in terms of antigen-binding specificity. Clearly the number of genes is far too small to account for most of this antibody diversity. Thus a germ-line hypothesis, whereby all antibodies are encoded by genes in germ-line cells, must be incorrect. In fact, the genes exist in separate coding sections and are assembled during B-lymphocyte maturation by a process called somatic recombination. A B lymphocyte can change the class of antibody being expressed by moving a new C gene segment into position after the recombined VDJ segment, deleting the intervening DNA. The process of class switching at the DNA level involves recombination between specific switch sequences located upstream of the heavy chain C segments.
