A key part of the immune system is the production of immunoglobulins (antibodies) by B cells to bind and inactivate specific foreign antigens. The body produces B cells with a wide range of antigen specificities in the immunoglobulin B cell receptor, one antigen specificity per cell. When the B cell receptor immunoglobulin binds antigen, that cell is activated to proliferate and create plasma cells secreting immunoglobulins to bind that specific antigen. B cell activation also creates memory cells with the same antigen specificity that do not actively secrete immunoglobulin but provide for rapid future immune responses to the same antigen. B cells are not activated by antigen on their own, but require interaction with helper CD4+ T cells to become activated and proliferate. The B cell first expresses immunoglobulin on the cell surface as the B cell receptor. If the B cell receptor immunoglobulin binds specific antigen, then the cell internalizes the antigen and presents it to T cells in MHC II, where it is recognized by the T cell receptor. In addition to the interaction between the T cell receptor and the B cell MHC-antigen, T cell interaction with the B cell involves additional positive and negative regulatory signals. CD40 interaction with CD40L and CD28 interaction with CD80 provide positive costimulatory signals that stimulate B cell activation and proliferation. T cell receptor activation induces expression of molecules like the CD40 ligand that modulate the B cell-T cell interaction. The CD40-CD40L interaction induces cytokine production and expression of other genes and alters the fate of the B cell involved in the interaction. If the interaction between CD40 and CD40L is prolonged, the B cell can be induced to become a memory cell rather than a plasma cell. Fas ligand binding to Fas between B and T cells may negatively modulate B cell activation, inducing apoptosis that limits B cell proliferation and activation. Cytokines like IL-2, IL-4 and IL-10 also play an important role in B cell activation.
