Other types of cells are found within the B-cell and T-cell areas. The B-cell zone contains a network of follicular dendritic cells (FDCs), which are concentrated mainly in the area of the follicle most distant from the central arteriole. FDCs have long processes that are in contact with B cells. FDCs are a distinct type of cell from the dendritic cells we encountered previously (see Section 1-3), in that they are not leukocytes and are not derived from bone marrow precursors; in addition, they are not phagocytic and do not express MHC class II proteins. FDCs are specialized for the capture of antigen in the form of immune complexes—complexes of antigen, antibody, and complement. The immune complexes are not internalized but remain intact on the surface of the FDC for prolonged periods of time, where the antigen can be recognized by B cells. FDCs are also important in the development of B-cell follicles.
T-cell zones contain a network of bone marrow-derived dendritic cells, sometimes known as interdigitating dendritic cells from the way in which their processes interweave among T cells. There are two major subtypes of these dendritic cells, distinguished by characteristic cell-surface proteins: one expresses the α chain of CD8, whereas the other is CD8-negative but expresses CD11b:CD18, an integrin that is also expressed by macrophages.
As in the spleen, the T cells and B cells in lymph nodes are organized into discrete T-cell and B-cell areas (see Fig. 9.1). B-cell follicles have a similar structure and composition to those in the spleen and are located just under the outer capsule of the lymph node. T-cell zones surround the follicles in the paracortical areas. Unlike the spleen, lymph nodes have connections to both the blood system and the lymphatic system. Lymph conducted to lymph nodes by afferent lymphatic vessels enters into the subcapsular space, which is also known as the marginal sinus, and brings in antigen and antigen-bearing dendritic cells from the tissues. T and B cells enter the lymph node via specialized blood vessels called high endothelial venules (HEVs) that are found in T-cell zones, as will be discussed further in Section 9-3.
The mucosa-associated lymphoid tissues (MALTs) are associated with the body’s epithelial surfaces, which provide physical barriers against infection. Peyer’s patches are part of the MALT and are lymph node-like structures interspersed at intervals just beneath the gut epithelium. They have B-cell follicles and T-cell zones (see Fig. 9.1), and the epithelium overlying them contains specialized M cells that are adapted to channel antigens and pathogens directly from the gut lumen to the underlying lymphoid tissue (see Section 1-16 and Chapter 12). Peyer’s patches and similar tissue present in the tonsils provide specialized sites where B cells can become committed to the synthesis of IgA. The mucosal immune system is discussed in more detail in Chapter 12.
9-2 The development of secondary lymphoid tissues is controlled by lymphoid tissue inducer cells and proteins of the tumor necrosis factor family.