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B cell and T cell lymphocytes interact with a variety of cells as part of their immune function, circulating and homing in on specific stimuli in tissues like inflammatory signals. The interaction of lymphocytes with other cell types like vascular endothelial cells requires transmembrane adhesion molecules like VLA-4, LFA-1, CD44, CD31 and L-selectin. Each of these lymphocyte adhesion molecules interacts with specific ligands expressed on cells like endothelial cells to moderate adhesion between cells. CD44 is an adhesion molecule with many roles, including interaction of leukocytes with endothelial cells in response to inflammation, with the most often studied ligand being hyaluronan, a component of the extracellular matrix. LFA-1, Leukocyte function-associated molecule 1, is an integrin composed of two subunits, CD11a and CD18. LFA-1 moderates adhesion of lymphocytes with other cells and also transduces signals into cells through interaction with ICAM-1 expressed on endothelial cells. Expression of ICAM-1 (CD54) is increased by inflammatory signals, increasing adhesion and permeation of lymphocytes into inflamed tissues. VLA-4 is another integrin expressed on lymphocytes that plays a role in adhesion through interaction with its adhesion partner on endothelial cells, VCAM-1. L-selectin appears to mediate adhesion through binding of a lectin-like domain to carbohydrates. PECAM-1 (CD31) is expressed on endothelial cells and is involved in angiogenesis as well as adhesion between cells. VAP-1 is an additional protein expressed by lymphocytes and involved in adhesion to vascular endothelial cells. (This definition may be outdated - see the DesignNote.)
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