ABSTRACT
Foreign invaders are recognized through specific and tight binding of the proteins of the immune system to molecules specific to the foreign organisms. The sites on foreign molecules that are recognized by the immune system are called antigenic determinants, and they interact with two different classes of antigen receptors produced by two major cell types of the immune system. Antibodies, also known as immunoglobulins, are produced by B cells, which are stimulated by antigen binding to secrete antibodies into the bloodstream (Figure 15.1a). Related molecules known as T-cell receptors, found on the surface of T cells, recognize and destroy virus-infected cells and play an important role in coordinating the immune response (Figure 15.1b).
