THE CASCADE OF THE IMMUNE-RESPONSE

The primary function of the immune system is to recognize foreign agen ts. To this end, the immune system has a vast repertoire of antigen re ceptors, whose diversity has been estimated to be on the order of 10(8 ), which means that the immune system can recognize and discriminate a mong 100 million different antigens. The moment an antigen is recogniz ed by the immune system, a series of sequential events take place at t he cellular level, resulting in the expansion of clones that react to the antigen. These clones focus on eliminating the antigen that activa ted them. Another aspect of the immune response is the production of m olecules that modulate the activation of accessory phagocytes that loc ate and eliminate the invading microorganism. Other cells that can cau se the cytolysis of cells infected by intracellular organisms also are activated. Ultimately, the immune response results in the production of antibodies, which either by themselves or by triggering other unspe cific amplifying mechanisms, help to eliminate pathogenic microorganis ms. The efficiency of the various immunologic components in locating a nd eliminating diverse infectious organisms varies according to the pr evailing host-parasite ratio. Some infections can be controlled only w hen specific antibodies are produced that act against the pathogen; th ese antibodies also protect individuals from reinfection. In other inf ectious diseases, such as tuberculosis, the antibodies cannot protect against or help eliminate the infectious agent. With other infections, such as the lepromatous form of leprosy, the presence of antibodies c an have a deleterious effect on the host.