Table 41.2 describes the different classes of immunoglobulins and their functions. Generally, a B cell makes only one class at a time. But class switching can occur, in which a B cell changes the immunoglobulin class it synthesizes. For example, a B cell making IgM can switch to making IgG.
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Early in its life, a B cell produces IgM molecules, which are the receptors responsible for its recognition of a specific antigen. At this time, the constant region of the heavy chain is encoded by the first constant region gene, the µ gene (see Figures 41.9 and 41.10). If the B cell later becomes a plasma cell during a humoral immune response, another deletion occurs in the cell’s DNA, positioning the variable region genes (consisting of the same V, D, and J genes) next to a constant region gene farther away on the original DNA molecule. Such a DNA deletion results in the production of a new immunoglobulin with a different constant region of the heavy chain, and therefore a different function (see Table 41.2). However, this immunoglobulin has the same variable regions—