CD31/PECAM-1 is a heavily glycosylated transmembrane homophilic adhesion protein that is highly expressed on endothelial cells and at a lower level on platelets, granulocytes, macrophages, dendritic cells, T and B cells, and NK cells. It is required for the transepithelial migration of leukocytes. CD31 signaling increases the activation threshold of T cells and inhibits dendritic cell development. The extracellular domain of CD31 is released during endothelial cell apoptosis. This fragment circulates in the serum of patients suffering from myocardial infarction, acute ischaemic stroke, and multiple sclerosis, conditions that involve tissue damage and endothelial cell apoptosis.