Adhesion to the vascular endothelium precedes or is a necessary prelude to leukocyte migration into the underlying tissue. Constitutive lymphocyte trafficking through lymphoid organs is controlled by tissue-specific interactions between molecules expressed on the surface of the lymphocyte (homing receptors) and ligands (vascular addressins) expressed on endothelial cells (HEV) within lymphoid tissues. Preliminary evidence suggests that lymphocytes may employ related but distinct interactions in their entry into some chronic sites of inflammation. Other leukocytes, such as neutrophils and monocytes, express molecules related or identical to lymphocyte homing receptors, and these molecules are exquisitely regulated by chemotactic factors and appear to be involved in the homing of these cells to inflamed tissues. In addition, inflammation in vivo induces increased endothelial cell adhesiveness for leukocytes that undoubtedly plays a key role in regulating leukocyte extravasation. Tissue- and inflammation-specific leukocyte/ endothelial cell adhesion molecules constitute attractive targets for suppression or manipulation of the early stages of tissue inflammation.
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