The ICAM family consists of three members. All are ligands for LFA1, and ICAM1 can also bind Mac1. However, the complete repertoire of other ligands has not yet been defined. The ICAMs are not functionally redundant, since they have quite distinct expression profiles and can bind LFA1 differentially. ICAM3 is constitutively and abundantly expressed by all leucocytes, whereas ICAM1 and ICAM2 are expressed at very low levels. ICAM1 is rapidly upregulated by inflammatory cytokines. A current working model for the interplay of the ICAMs is that ICAM3 is the major ligand for LFA1 in the initiation of immune responses and that ICAM1 takes over after these early stages. ICAM3 not only functions as an adhesion molecule, but also is clearly emerging as a potent signalling molecule.