| The specialized sites of T cell activation are colored purple; the sites of B cell activation are blue. In lymph nodes (a), spleen (b) and the Peyer's patches of the small intestine (c), the B cell zones are seen as defined follicles; functionally equivalent areas, not all as well defined, are found in all mucosal lymphoid tissues. Proliferating B cells are concentrated in the germinal centers. The T cell regions (purple) are adjacent to the B cell follicles. In the spleen, these form the periarteriolar lymphatic sheaths (PALS) and are surrounded by a region known as the marginal zone, which we describe in the next section. In the lymph nodes, the T cell zones are separated from the medullary sinus, which forms the exit route into the efferent lymphatic vessels, by a region known as the medullary cords, which are rich in macrophages and plasma cells secreting large quantities of antibody. |
