Constant region genes

In humans, there are nine functional heavy chain constant region genes (CH, C8, C73, C.,1, CJ, Cy2, C74, Q and Ca2; Figure 1A), one k light chain constant region gene (CK; Figure IB) and four functional \ light chain constant region genes (Cxl, Cx2, Ck3

and C\7; Figure 1C) (see Table 1). The constant region genes expressed by a particular B cell determine the immunoglobulin's isotvpe. Initially each B cell expresses the C^ gene in association with one of the five light chain constant region genes (producing the immunoglobulin M (IgM) isotvpe). Note that k and X light chains are rarely or never expressed in the same B cell. The mechanism that enables immunoglobulins other than the IgM isotvpe to he expressed is known as 'class switching'. This involves a recombination event at the DNA level between 'switch' sequences upstream of the C(l gene and those upstream of the Cy, Ce and CK genes, resulting in the expression of the IgG, IgE and IgA isotypes, respectively (this deletes the intervening region of DNA). In the case of the IgD isotype, the Q, gene is sufficiently close to the C^ gene to be spliced into the [,, segment at the RNA level (see Figure 1A). Thus, IgD is an exception in that it can be coexpressed along with IgM by the same B cell.

Table 1 Number of functional variable and constant region genes
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