CD50 is a 120-140 kD type I membrane protein also known as intercellular adhesion molecule 3 (ICAM-3). It is broadly expressed on leukocytes, endothelial cells, and Langerhans cells. CD50 is a member of the Ig superfamily. It provides costimulatory signal for T cells and regulates cell adhesion through interaction with LFA-1 (CD11a/CD18, αL/β2) integrin.