Cell communication is essential to development, tissue organization and function of all multicellular organisms. Cells communicate with each other and with their environment via soluble mediators and during direct contact. In the immune system leukocytes are involved in a host of cell-soluble mediator, cell-cell and cell-matrix interactions. For instance, an acute inflammatory response is initiated by release of soluble substances at the site of injury that direct leukocyte migration and localization. The circulating leukocytes arriving at the site of injury must directly adhere to the endothelium and basement membrane during extravasation from the circulation. Once in the tissue, leukocytes will migrate in response to gradients of chemoattractants and to adhesive gradients. The leukocytes will adhere and de-adhere to a variety of cells, finally find infected or antigen-presenting cells, adhere to them and deliver the appropriate immunological response.

Discovery of several cell surface receptors involved in cell-cell, cell-substrate, or cell-soluble ligand binding has been a key factor in understanding the mechanisms underlying inflammatory and immune phenomena. Three protein families, the immunoglobulin (Ig) family (Williams & Barclay 1988, Williams 1987), the integrin family (Springer 1990, Kishimoto et al. 1989a, Hynes 1987, Ruoslahti & Pierschbacher 1987, Hemler 1988), and the recently designated selectin family (Tedder et al. 1989, Stoolman 1989, Springer 1990), have been described that are extensively involved in a network of cell-cell and cell-matrix interactions in the immune system. Engagement of these surface receptors can transduce a signal leading to cellular events that change the phenotype, movement, gene expression, or activation state of the cell. On the other hand, cytoplasmic signals regulate the functional activity and surface expression of these receptors. These receptors thus mediate a dialogue, transferring information in