When a T_H cell survives the selection processes and binds to an antigen-presenting cell, it releases cytokines that activate the T_H cell to proliferate, producing a clone of T_H cells with the same specificity. The steps to this point constitute the activation phase of the humoral immune response, and they occur in the lymphoid tissues. Next comes the effector phase, in which the T_H cells activate previously unexposed B cells with the same specificity to produce antibodies.

B cells are also antigen-presenting cells. B cells take up antigens bound to their surface immunoglobulin receptors by endocytosis, break them down, and display antigenic fragments on class II MHC proteins. When a T_H cell binds to the displayed antigen–MHC II complex, it releases cytokines that cause the B cell to produce a clone of plasma cells and memory cells (Figure 41.13A). Finally, the plasma cells secrete antibodies, completing the effector phase of the humoral immune response.