ABSTRACT
Three unlinked gene groups encode immunoglobulins—one for k chains, one for λ chains, and one for H chains, each on a different chromosome (Table 1). Within each of these gene groups on the chromosome there are multiple coding regions (exons) that recombine at the level of DNA to yield the DNA coding for a binding site. In a mature B cell or plasma cell, the DNA encoding the V region for the H chain of a specific antibody consists of a continuous uninterrupted nucleotide sequence. In contrast, the DNA in a germ line cell (or non-B cell) for this V region exists in distinct DNA segments, exons, separated from each other by regions of noncoding DNA (Figure 1). The exons encoding the V region of the H chain are V segment (encoding approximately the first 102 amino acids), D segment (encoding 2–4 amino acids), and J segment (encoding the remaining 14 or so amino acids in the V region). For L chains there are only V-segment (encoding the first approximately 95 amino acids) and J-segment (encoding the remaining 13 or so amino acids) exons. In each gene group, there are from 30–65 functional V-segment genes. The D and J regions are between the V and C regions on the chromosome and there are multiple different genes for each, but fewer in number than those encoding the V segment. Thus, DNA segments that ultimately encode the binding site of antibodies have to be moved over distances (translocated) on the chromosome to form a DNA sequence encoding the V region (gene “rearrangement”).
