Figure 1. The human lymphoid system. The lymph nodes, indicated as dots, and the lymphatics, as dashed lines, return cells to the circulation near the heart.

Figure 2. The thymus is enclosed in a connective tissue capsule and is divided into an outer cortex and an inner medulla. T-cell education refers to the ability of the T cell to recognize MHC (positive selection) and self antigens (negative selection) on thymic epithelial cells.

Figure 3. The lymph node is enclosed in a connective tissue capsule. The lymph, including cells, enters from the afferent lymphatic into the subcapsular sinus and then percolates through the node via a series of channels known as cortical and medullary sinuses. Lymphocytes also enter the lymph node from the blood in high endothelial venules in the paracortex and leave via the efferent lymphatic. The development of an immune response coincides with the appearance of germinal centres.

Figure 4. The Peyer patch comprises many lymphoid follicles, which cause a thickening in the wall of the intestine. Antigen from the intestinal lumen enters via the dome region, where it initiates events that lead to the proliferation of B lymphocytes in the follicle and the eventual generation of an IgA antibody response at mucosal sites.

Figure 5. The white pulp comprises lymphoid aggregates that associate with central arterioles at numerous sites throughout the spleen. The white pulp in the diagram includes the region inside the dotted line; the surrounding regions are known as the red pulp.

Figure 6. Pathways of lymphocyte movement between the tissues of the immune system. The bone marrow is the source of lymphoid progenitors. T-lymphocyte progenitors need a period of time in the thymus before being able to function as mature T lymphocytes. Mature T and B lymphocytes are converted by the blood (represented by the pink region) to the various lymphoid tissues. Lymphocytes that enter the spleen can return directly to the blood if they are not activated by antigen. Those lymphocytes that enter the Peyer patches (PP), however, exit via lymphatic vessels, called lacteals, that carry them in the lymph to a lymph node. Thus, the lymph node receives cells from multiple sources, including the lacteals from the intestine and afferent lymphatics from skin, muscle and most organs. Lymphocytes also enter the lymph node directly from the circulation. Lymphocytes that do not become part of an immune response leave the lymph node via the efferent lymphatic and return to the blood in the left subclavian vein.