The thymus is the primary lymphoid organ where immature T cells, thymocytes, mature into the specific subpopulations of T cells (e.g. helper or cytotoxic T cells) and gain central tolerance. The development of central tolerance is crucial so that these cells do not become activated by self-antigens.

Examine this section of thymus and the image at the right, noting the capsule, connective tissue septa, and organization of lobes into a basophilic cortex and a paler staining medulla.

Identify the thymic epithelial cells (also called epithelial-reticular cells or epitheliocytes), which make up the framework of the thymus. In the cortex as seen below, these cells are larger than the lymphocytes, and their nuclei contain prominent nucleoli. In the medulla, identify the thymic corpuscles or Hassall corpuscles, which are large collections of thymic epithelial cells. Thymic epithelial cells function to 1) form a blood-thymus barrier to shield developing thymocytes from exposure to antigens, 2) compartmentalize the thymus to sequester immature and mature thymocytes, in the cortex and medulla, respectively, 3) provide a structural and supportive framework for the developing thymocytes (the thymus lacks reticular fibers), and 4) present self and foreign antigens to developing thymocytes during positive and negative selection.

Last, examine a section of involuted thymus, and note the accumulation of adipocytes and depletion of lymphocytes.

Next let's look at lymph nodes.