Human CD31 is an adhesion molecule expressed on platelets, endothelial cells, leukocytes and their bone marrow precursors. CD31 plays a role in homophilic adhesion and heterophilic transendothelial migration. Antibody TP1/15 reacts with domain 1 of CD31 and blocks homophilic interaction and heterophilic transendothelial migration. Expressed diffusely by monocytes, neutrophils, and NK cells. Expressed on subsets of T cells.