Lymphocytes allow the immune system to recognize and respond specifically to a host of foreign antigens. Three known types of lymphocytes exist that are characterized by three different types of clonally distributed receptors, which they use to recognize antigen.

B lymphocytes have receptors consisting of immunoglobulins. These cells usually recognize foreign material in its native form; for example during a virus infection, B cells bind the free virus and produce serum immuno globulins that have the same specificity as the membrane bound receptor form. These antibodies bind cell-free virus and inhibit its infectious activity, or stimulate its uptake by phagocytic cells such as macrophages, thereby causing the clearance of the infectious agent from the animal (Amzel et al 1974; Richards et al 1975).