The ICAMs are cell-surface ligands for the leukocyte integrins and are crucial in the binding of lymphocytes and other leukocytes to certain cells, including antigen-presenting cells and endothelial cells. They are members of the immunoglobulin superfamily. ICAM-1 is the most prominent ligand for the integrin CD11a:CD18 or LFA-1. It is rapidly inducible on endothelial cells by infection, and plays a major role in local inflammatory responses. ICAM-2 is constitutively expressed at relatively low levels by endothelium. ICAM-3 is expressed only on leukocytes and is thought to play an important part in adhesion between T cells and antigen-presenting cells, particularly dendritic cells.