CD81 is a 26 kD non-glycosylated member of the tetraspanin superfamily (TM4SF), also known as TAPA-1 (target of an antiproliferative antibody). CD81 is expressed on T and B cells, NK cells, monocytes, dendritic cells, thymocytes, endothelial cells, and fibroblasts. It also has low levels of expression on granulocytes. CD81 induces B cell adhesion via VLA-4 integrin and has been shown to play a role in early T cell development. CD81 associates with several other cell-surface proteins in a multimolecular complex, including CD19, CD21, CD20, CD37, CD53, and CD82 in B cells, and CD4, CD8 and CD82 in T cells.