B-R1 plays an important role in providing a costimulatory signal in the immune response and mediates adhesion between leucocytes. It is constitutively expressed on resting antigenpresenting cells, including dendritic cells, and appears to be important in the initial interactions
between T cells and cells leading to T cell activation. Cross-linking of CD50 on T cells with monoclonal antibodies can mobilize intracellular calcium [2], activate tyrosine phosphorylation, and stimulate T cell adhesion and proliferation [3,4]. CD50, like CD54 and CD102 is a ligand for
the leucocyte integrin LFA-1 (CD11a/CD18) [5]. It is a highly glycosylated type I membrane protein.