Coordinated cell-cell and cell-substratum interactions are crucial for the differentiation, organization and diverse somatic functions of multicellular organisms. Several families of molecules have evolved to serve such specialized purposes such as cell migration in development and in the immune response. One of these families, the integrins, plays a prominent role in the attachment and detachment of cells to their surrounding matrix, the adhesion of platelets to fibrinogen, the coupling of lymphocytes to antigen presenting cells and the phagocytosis of complement opsonized targets by myelomonocytic phagocytes in the immune system. At least 21 different integrin receptors have been characterized to date, all of which are heterodimeric transmembrane proteins that comprise an α subunit which is non-covalently associated with a β subunit at the cell surface. The family was originally characterized by structurally related β subunits that formed a series of heterodimers with distinct α subunits. However, recent findings indicate that there are several important exceptions and that more elaborate schemes are required to describe the systematic structural organization of these receptors. A number of comprehensive reviews on this issue have appeared recently (Diamond and Springer 1994; Hemler 1990; Hynes 1992; Springer 1990). Integrins bind to a diverse array of ligands including extracellular matrix (ECM) proteins, plasma proteins which mediate hemostasis and complement activation (fibrinogen and ic3b), and integral membrane proteins.