ABSTRACT
The other component of specific response to infection is known as cell-mediated immunity. From the progenitor stem cell, the lymphopoietic cells enter the thymus gland via the bloodstream at about the eighth week of gestation and differentiate into lymphocytes. The thymus gland is derived from the epithelium of the fourth pharyngeal pouches by the sixth week of gestation. A hormone, thymosin, may be operative in the generation of lymphocytes. The lymphocytes that emerge from the thymus acquire new surface antigen markers, which may be antigen recognition units. The lymphocytes travel via lymphatics and the bloodstream, and populate very specific areas in the lymph nodes called "thymic-dependent" areas and the periarterial areas of the spleen. These pools of lymphocytes have a very long life. The whole complex is referred to as the T cell system (21). The DiGeorge syndrome, presenting with hypocalcemia and repeated viral and fungus infections, is explained by the development of the thymus and parathyroid glands from the same third and fourth pharyngeal pouch (17). If a newborn infant presents with hypocalcemia not explained on the basis of other problems, an evaluation of thymic-dependent immunity would be in order. Cellular immunity must be active in the fetus, as the phytohemagglutinin (PHA) test that stimulates the thymic cells is positive from about 14 weeks. Also, maternal lymphocytes that enter the fetal circulation should mount a graft-versus-host reaction, if they were not rejected by a competent system in the fetus that rejects them.
