Overview of the Thymic-Dependent Immune System

In order to fully understand the complex biochemical mechanisms by which the thymus and its hormones control the expression of immunity, it is first necessary to review briefly the organization of the thymus-dependent (T cell) immune system (Fig. I). The predominant cells of the peripheral lymphoid tissue (i.e., spleen, lymph nodes) includes both B and T lymphocytes. In the presence of foreign antigens, B lymphocytes differentiate into plasma cells, which in turn synthesize antibody thus, B cells make up the humoral arm of the immune system. In contrast, T lymphocytes are responsible for mediating all the classical cellular immune responses such as delayed type hypersensitivity skin responses, organ transplant rejections, and sensitized antitumor immunity as well as immunity toward various viral, fungal, and protozoal pathogens (Reinherz and Schlossman, 1980). 

The thymus gland is required for the normal maturation and differentia- 

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