All ICAM proteins are type I transmembrane glycoproteins，contain 2-9 immunoglobulin-like C2-type domains，and bind to the leukocyte adhesion LFA1 protein. ICAM3 is constitutively and abundantly expressed by all leucocytes and may be the most important ligand for LFA1 in the initiation of the immune response. It functions not only as an adhesion molecule，but also as a potent signalling molecule. Upon stimulation by a physiological stimulus it becomes rapidly and transiently phosphorylated on serine residues. N-glycosylated; glycans consist of a mixture of tri- and tetra-antennary complex-type chains and high-mannose chains.