CD43, also known as Ly-48, Leukosialin, Sialophorin, Leukocyte Sialoglycoprotein, and gp115, is a large single chain of type I transmembrane glycoprotein with abundant O-glycosylation and sialylation sites. Due to variable glycosylation and sialylation, two isoforms of CD43 have been identified. The 115 kD glycoform of CD43 is expressed on most hematopietic cells including T lymphocytes, NK cells, monocytes, granulocytes, platelets, and CD5+ B cells. It is not present on resting B cells and erythrocytes. While the 130 kD glycoform is thought to be activation-associated form primarily expressed on myeloid cells, pre-B cells, and activated T cells. It has been reported that CD43 binds to CD54 and Siglec-1. CD43 plays dual roles in cell adhesion and anti-adhesion, as well as costimulation of T cell activation and survival, and induction of apoptosis of T cells and hematopoietic progenitors. The 1B11 antibody reacts with the activation-associated glycoform of CD43. The epitope recognized by 1B11 is also shared with desialylated CD45RB. This antibody is useful for differentiation of effector CD8 T cells and memory T cells.