Comparison of murine lymphoid compartments and the migration pathways of lymphocytes into the splenic white pulp and the lymph nodes. Spleen: Lymphocytes enter the white pulp of the spleen from the marginal zone and entry is mediated by signaling through chemokine receptors. B cells are attracted to the B-cell follicles by CXC-chemokine ligand 13 (CXCL13), whereas T cells are directed to the T-cell zone by responding to CC-chemokine ligand 19 (CCL19) and CCL21. It is unclear how lymphocytes eventually leave the white pulp. Lymph node: Few lymphocytes enter the lymph node from the afferent lymphatic vessels. Most lymphocytes enter through specialized blood vessels that are known as high endothelial venules (HEVs) and then migrate to the B-cell follicles or the T-cell zone, which again is regulated by CXCL13, CCL19, and CCL21, respectively. Lymphocytes exit lymph nodes in efferent lymphatic vessels and then reenter the bloodstream from the lymph. (Figure and legend reprinted from Mebius and Kraal.¹⁴ )