Organized Lymphoid Tissue
The role of the bone marrow in hematopoiesis and of the thymus in T‐cell development will be discussed in Chapter 10. As already discussed, the MALT deals with antigens present at mucosal surfaces. In contrast, the lymph nodes receive antigen either draining directly from the tissues or carried by dendritic cells, and the spleen monitors the blood.
Figure 6.11 The distribution of major lymphoid tissues throughout the body.
Figure 6.12 Traffic and recirculation of lymphocytes through encapsulated lymphoid tissue and sites of inflammation. Blood‐borne lymphocytes enter inflamed tissues when they recognize upregulated adhesion molecules on the blood vessel endothelium and enter lymph nodes by passing through the high‐walled endothelium of the postcapillary venules (HEV). They leave via the draining lymphatics. The efferent lymphatics join to form the thoracic duct, which returns the lymphocytes to the bloodstream. In the spleen, which lacks HEVs, lymphocytes enter the lymphoid area (white pulp) from the arterioles, pass to the sinusoids of the erythroid area (red pulp) and leave by the splenic vein (see Figure 6.16b). Traffic through the mucosal immune system was illustrated in Figure 6.7.