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.