The BNI3 monoclonal antibody specifically reacts with human CD152, the Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4). CTLA-4 is expressed on activated CD28+ T cells, and binds the B7 family members B7-1 (CD80) and B7-2 (CD86). The structure of CTLA-4 is similar to the structure of CD28, but the two molecules seem to have opposite roles on the T lymphocytes. CTLA-4 inhibits the progression of T cell activation, while CD28 stimulates it. This result explains the stimulating role that the immobilization of BNI3 plays on the T lymphocytes proliferation induced by CD28.