Final answer:
Once a B cell is activated, it undergoes clonal proliferation and differentiation into plasma cells. Plasma cells secrete large quantities of antibodies, initially IgM, with the same specificity as the B cell receptors (BCRs). Cytokines secreted by T² cells then stimulate plasma cells to switch production to other antibody classes through class switching.
Step-by-step explanation:
Once a B cell is activated, it undergoes clonal proliferation and daughter cells differentiate into plasma cells. Plasma cells are antibody factories that secrete large quantities of antibodies. After differentiation, the surface BCRs disappear and the plasma cell secretes pentameric IgM molecules that have the same antigen specificity as the BCRs.
After initial secretion of IgM, cytokines secreted by T² cells stimulate the plasma cells to switch from IgM production to production of IgG, IgA, or IgE. This process, called class switching or isotype switching, allows plasma cells cloned from the same activated B cell to produce a variety of antibody classes with the same epitope specificity. Class switching is accomplished by genetic rearrangement of gene segments encoding the constant region, which determines an antibody's class. The variable region is not changed, so the new class of antibody retains the original epitope specificity.