Small IgM+, IgD+ B cells can switch to the production of other classes (and subclasses) of immunoglobulin (Ig) (1-4). This switch can be induced by either antigenic (5, 6) or mitogenic stimulation (7). The nature of the antigen (8), the site of antigen injection (9), and the nature of the T cell response (8, 10-13) can determine the class of Ig produced in vivo. In particular, the switch in isotype synthesis isrelatively T cell dependent (8, 10, 11). Furthermore, T cells can influence the subclassg- of IgG secreted by B cells (12, 13). However, in low density cell cultures in thett presence of lipopolysaccharide (LPS),1 IgG secretion by murine B cells may not3 require T cells (5, 14-17).
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