Mature B lymphocytes migrate from one secondary lymphoid organ to the next in search of antigen. Naive B cells reside in and circulate through the follicles of peripheral lymphoid organs (the spleen, lymph nodes, and mucosal lymphoid tissues) in search of their cognate antigen (see Chapters 2 and 3). Most B cells enter follicles and are called follicular B cells or recirculating B cells. Entry into the follicles is guided by the chemokine CXCL13 secreted by follicular dendritic cells and other stromal cells in the follicle. CXCL13 binds to the CXCR5 chemokine receptor on recirculating naive B cells and attracts these cells into the follicles. As we discuss later, the same chemokine-receptor pair is also important during immune responses because it can attract a subset of activated T cells to the follicle. Naive follicular B cells survive for limited periods until they encounter antigen (see Chapter 2). Follicular B cell survival depends on signals from the BCR as well as on inputs received from a tumor necrosis factor (TNF) superfamily cytokine called BAFF (B cell-activating factor of the TNF family, also known as BLyS, for B lymphocyte stimulator), which provides maturation and survival signals through the BAFF receptor. BAFF and a related ligand, APRIL, can activate two other receptors, TACI and BCMA, which participate in later stages of B cell activation and differentiation (and will be discussed later). These cytokines are produced mainly by myeloid cells in lymphoid follicles and in the bone marrow.
Antigen may be delivered to naive B cells in lymphoid organs in different forms and by multiple routes. Antigens that enter by crossing an epithelial barrier as well as antigens in the circulation are capable of activating B cells and are brought to B cell zones by several mechanisms (Fig. 11-4).
• Most antigens from tissue sites are transported to lymph nodes by afferent lymphatic vessels that drain into the subcapsular sinus of the nodes. Soluble antigens, generally smaller than 70 kD, may reach the B cell zone through conduits that extend between the subcapsular sinus and the follicle and interact directly with specific B cells.
• Subcapsular sinus macrophages capture large microbes and antigen-antibody complexes and deliver these to follicles, which lie under the sinus.
• Many relatively large antigens that enter the node through afferent lymphatic vessels are not captured by subcapsular sinus macrophages but are too large to
Primary antibody response
Secondary antibody response
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