Clone REA612 recognizes the human CD28 antigen, a type I transmembrane protein, which is highly expressed on CD3+
thymocytes, most peripheral thymocytes, and plasma cells but not in less mature B cells. Binding of CD28 to its ligands CD80 or CD86 costimulates T cell effector function and T cell–dependent antibody production in vitro
and in vivo
. In the presence of antibodies directed against CD2 and CD3, CD28 antibodies stimulate T cell proliferation and cytokine production.