Fig. 1.22 Organization of a lymph node. As shown at left in the diagram of a lymph node in longitudinal section, a lymph node consists of an outermost cortex and an inner medulla. the cortex is composed of an outer cortex of B cells organized into lymphoid follicles and of adjacent, or paracortical, areas made up mainly of t cells and dendritic cells. When an immune response is under way, some of the follicles—known as secondary lymphoid follicles— contain central areas of intense B-cell proliferation called germinal centers. these reactions are very dramatic, but eventually die out as germinal centers become senescent. lymph draining from the extracellular spaces of the body carries antigens in phagocytic dendritic cells and phagocytic macrophages from the tissues to the lymph node via the afferent lymphatics. these migrate directly from the sinuses into the cellular parts of the node. lymph leaves via the efferent lymphatics in the medulla. the medulla consists of strings of macrophages and antibody-secreting plasma cells known as the medullary cords. naive lymphocytes enter the node from the bloodstream through specialized postcapillary venules (not shown) and leave with the lymph through the efferent lymphatic. the light micrograph (right) shows a transverse section through a lymph node, with prominent follicles containing germinal centers. magnification ×7. Photograph courtesy of n. rooney.
diffusely distributed in the surrounding paracortical areas, also referred to as the deep cortex or T-cell zones (see Fig. 1.22). Lymphocytes migrating from the blood into lymph nodes enter the paracortical areas first, and because they are attracted by the same chemokines, antigen-presenting dendritic cells and macrophages also become localized there. Free antigen diffusing through the lymph node can become trapped on these dendritic cells and macrophages. This juxtaposition of antigen, antigen-presenting cells, and naive T cells in the T-cell zone creates an ideal environment in which naive T cells can bind their specific antigen and thus become activated.
As noted earlier, activation of B cells usually requires not only antigen, which binds to the B-cell receptor, but also the cooperation of activated helper T cells, a type of effector T cell. The location of B cells and T cells within the lymph node is dynamically regulated by their state of activation. When they become activated, T cells and B cells both move to the border of the follicle and T-cell zone, where T cells can first provide their helper function to B cells. Some of the B-cell follicles include germinal centers, where activated B cells are undergoing intense proliferation and differentiation into plasma cells. These mechanisms are described in detail in Chapter 10.
In humans, the spleen is a fist-sized organ situated just behind the stomach (see Fig. 1.18). It has no direct connection with the lymphatic system; instead, it collects antigen from the blood and is involved in immune responses to blood-borne pathogens. Lymphocytes enter and leave the spleen via blood vessels. The spleen also collects and disposes of senescent red blood cells. Its organization is shown schematically in Fig. 1.23. The bulk of the spleen is composed of red pulp, which is the site of red blood cell disposal. The lymphocytes surround the arterioles running through the spleen, forming isolated
Principles of adaptive immunity.