s and toxins before they are able to disperse (6:405-413).
The lymphoid tissue is responsible for producing lymphocytes. Unlike many other cell types, lymphocytes initially lack a specific function. The cells originate in the embryo from lymphocytic stem cells. Before the descendants of these stem cells migrate to the lymphoid tissue, however, they must first undergo preprocessing. T lymphocytes are preprocessed in the thymus gland (hence the name T lymphocytes). In contrast, B lymphocytes--which were first discovered in birds--are preprocessed in the avian bursa of Fabricius (hence the name B lymphocytes). In mammals, however, this structure does not exist. While the preprocessing area for B lymphocytes in humans remains unknown, it is thought to occur in the fetal liver (6:405-413).
Once lymphocytes have been processed, they circulate freely in the blood and eventually filter into the tissues. With time, the cells enter the lymph and are transported to the lymphoid tissue. The fine interwoven meshwork of collagen fibers within this tissue subsequently traps the cells (6:405-413).
T lymphocytes and B lymphocytes mediate immunity in two different ways. The T lymphocyte system provides cellular immunity, and the B lymphocyte system is responsible for humoral immunity. Cellular immunity--which is also known as lymphocytic immunity--involves the formation of large numbers of highly specialized lymphocytes. These specifically sensitized cells have the ability to attack and destroy foreign agents. In contrast, with humoral immunity, the B lymphocytes produce antibodies. These globulin molecules then attack the invading agents (6:405-413).
The sensitized lymphocytes and antibodies formed by the lymphoid tissues react with high specificity against invading agents. It is believed that "literally hundreds or thousands of different types of precursor lymphocytes preexist in the lymph nodes (6:405-413)." Each of t...