Intercellular adhesion molecule 1 (ICAM-1, also known as CD54) is a transmembrane glycoprotein of the immunoglobulin superfamily and is critical for the firm attachment and transmigration of leukocytes out of blood vessels and into tissues. ICAM-1 is expressed by several cell types, typically on endothelial cells and cells of the immune system, and its expression can be up-regulated by various stimuli, including TNF-α, INF-γ, IL-1 and thrombin. It is a ligand for LFA-1 and Mac-1, serves as a receptor for rhinovirus, and is one of several receptors used by Plasmodium falciparum. ICAM-1 can exist as membrane-bound form (mICAM-1) and soluble form (sICAM-1). The sICAM-1 arises from alternative splicing and proteolysis of mICAM-1 and appears to be associated with leukocyte activation to produce LFA-1.