Leukocyte subsets are recruited from the blood to lymphoid and non-lymphoid tissues via a multi-step process that involves distinct adhesive and activation steps. Chemokines, a family of chemotactic cytokines that signal through G-protein-coupled receptors, play critical roles in regulating the leukocyte recruitment cascade. Chemokines can be transported and immobilized on the surface of vascular endothelial cells, where they activate leukocyte subsets expressing specific receptors. Activation signals induce firm adhesion of rolling leukocytes by rapidly upregulating integrin affinity and/or avidity. Chemokines can also direct migration of adherent cells across the endothelium, and control segregation of cells into specific microenvironments within tissues. The regulated expression of chemokines and their receptors is a critical determinant for homing of specialized lymphocyte subsets, and controls both tissue and inflammation-specific immune processes.
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