CD2 is a 50 kD type I transmembrane glycoprotein also known as LFA-2, T11, and sheep red blood cell receptor (SRBC-R). This immunoglobulin superfamily member is expressed on thymocytes, T lymphocytes, NK cells, and thymic B cell subsets. The major ligand for CD2 is CD58 (also known as LFA-3). CD2 has also been reported to bind CD48, CD59, and CD15. CD2 plays a critical role in alternative T cell activation, T cell signaling, and cell-cell adhesion.