ICAM-1 facilitates a variety of both afferent and efferent immune responses that require intercellular contact and collaboration, including T helper responses, T-dependent B cell responses, antigen-induced T cell proliferation, cell-mediated cytotoxicity, and the binding of leukocytes to vascular endothelium prior to extravasation into inflamed tissue. In these events, the binding of ICAM-1 to LFA-1 acts as an antigen-independent, generalized 'accessory' adhesion mechanism that holds cells in close proximity long enough for other receptor/ligand interactions (MHC/Ag receptor, cytokines, etc.) and cellular functions to take place.

The phenotypes of ICAM-1-deficient mice show that ICAM-1 is not essential for leukocyte development and maturation. However, these mice display impaired contact hypersensitivity, defective neutrophil migration, and their cells are defective stimulators of mixed lymphocyte responses. In addition, ICAM-1-deficient mice are resistant to septic shock induced by either superantigen or endotoxin, and this resistance appears to reflect defects in T cell activation by superantigen and decreased neutrophil infiltration into the liver stimulated by endotoxin.

In addition to its role in the immune system,

ICAM-1 has been subverted by some human pathogens for their own use; the major group of human rhinoviruses (causative agents of the common cold) use ICAM-1 as an obligate receptor for cell entry and infection, and red blood cells infected with the malarial parasite Plasmodium falciparum bind to ICAM-1 expressed on endothelial cells.

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