This antibody reacts with the CD28-antigen, which is expressed on approximately 29 % of CD4+. CD28 mediates cell adhesion trough the two ligands, CD80, expressed on activated B cells. Cross-blooking of CD28 induces T cell activation, suggesting an important role for CD28 in the interaction between B and T cells. This antibody recognizes an homodimeric transmembrane glycoprotein present on most mature T cells, thymocytes and plasma cells.