Molecular mechanisms that control leukocyte extravasation: the selectins and the chemokines

Attachment of leukocytes to the blood vessel wall initiates leukocyte extra vasation. This enables leukocytes to migrate to and accumulate at sites of tissue injury or infection where they execute host-defense mechanisms. A se ries of vascular cell adhesion molecules On leukocytes and on endothelial c ells mediate leukocyte attachment to the endothelium in a stepwise process. A large panel of about 40 known human chemokines is able to specifically a ctivate certain leukocytes and attract them to migrate across the endotheli al barrier and within tissue. The specific combination of molecular signals provided by the diversity of cytokines, adhesion molecules, and chemokines regulates the specificity and selectivity of the recruitment of certain su bpopulations of leukocytes in vivo. This review will focus on selectins and chemokines which initiate the cell contact and regulate activation and che moattraction of leukocytes.