As discussed in Chapter 8, the primary lymphoid organs—the thymus and bone marrow—are the tissue sites where antigenic receptor repertoires of T and B cells, respectively, are selected. Adaptive immune responses are initiated in the secondary lymphoid organs—lymph nodes, spleen, and the mucosa-associated lymphoid tissues (MALTs) such as the Peyer’s patches in the gut. The architecture of these tissues is similar throughout the body and is structured to provide a crossroads for the interaction of rare clonal precursors of recirculating T and B cells with their cognate antigens—whether delivered by dendritic cells in the case of T cells, or as free antigens in the case of B cells. In view of the rarity of naive T cells that recognize a specific peptide:MHC complex—roughly 50–500 cells in the entire immune repertoire of approximately 100 million T cells in the mouse—and the large area over which an infectious agent can invade, the antigens derived from the pathogen, or in some instances the pathogen itself, must be brought from sites of entry to secondary lymphoid organs to facilitate their recognition by lymphocytes. In this part of the chapter we shall first consider the development and structure of secondary lymphoid organs that enable these interactions. We shall then discuss how naive T cells are directed to exit the blood and enter the lymphoid organs. This will be followed by considering how dendritic cells pick up antigen and travel to local lymphoid organs, where they can both present antigen to T cells and activate them.
9-1 T and B lymphocytes are found in distinct locations in secondary lymphoid tissues.
The various secondary lymphoid organs are organized roughly along the same lines (see Chapter 1), with distinct areas in which B cells and T cells are concentrated—the B-cell and T-cell zones. They also contain macrophages, dendritic cells, and nonleukocyte stromal cells. In the case of the spleen, which is specialized for the capture of antigens that enter the bloodstream, the lymphoid tissue component is called the white pulp (Fig. 9.1).