Naive B cells use the same basic mechanisms as do naive T cells to home to secondary lymphoid tissues throughout the body, which enhances their likelihood of responding to microbial antigens in different sites. Immature B cells leave the bone marrow through the blood and enter the red pulp of the spleen, migrate to the periphery of the white pulp, and then, as they mature further, move into the white pulp in response to a chemokine called CXCL13, which binds to the chemokine receptor CXCR5 expressed by the B cell. Once the maturation is completed within the white pulp, naive follicular B cells reenter the circulation and home to lymph nodes and mucosal lymphoid tissues. Homing of naive B cells from the blood into lymph nodes involves rolling interactions on HEVs, che-mokine activation of integrins, and stable arrest, as described earlier for naive T cells. Once naive B cells enter the stroma of secondary lymphoid organs, they migrate into follicles, the site where they may encounter antigen and become activated. This migration of naive B cells into follicles is mediated by CXCL13, which is produced in follicles and binds to the CXCR5 receptor on naive B cells. Homing of naive B cells into Peyer's patches involves CXCR5 and the integrin a4p7, which binds to MadCAM-
I. During the course of B cell responses to protein antigens, B cells and helper T cells must directly interact, and this is made possible by highly regulated movements of both cell types within the secondary lymphoid organs. These local migratory events, and the chemokines that orchestrate them, will be discussed in detail in Chapter
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