A substance being studied in the treatment of some types of cancer. Tremelimumab binds to a protein called CTLA-4, which is found on the surface of T cells (a type of white blood cell). It may block CTLA-4 and help the immune system kill cancer cells. Tremelimumab is a type of monoclonal antibody and a type of immunomodulatory agent. OR A human immunoglobulin (Ig) G2 monoclonal antibody directed against the human T-cell receptor protein cytotoxic T-lymphocyte-associated protein 4 (CTLA4), with potential immune checkpoint inhibitory and antineoplastic activities. Tremelimumab binds to CTLA4 on activated T-lymphocytes and blocks the binding of the antigen-presenting cell ligands B7-1 (CD80) and B7-2 (CD86) to CTLA4, resulting in inhibition of CTLA4-mediated downregulation of T-cell activation. This promotes the interaction of B7-1 and B7-2 with another T-cell surface receptor protein CD28, and results in a B7-CD28-mediated T-cell activation that is unopposed by CTLA4-mediated inhibition. This leads to a cytotoxic T-lymphocyte (CTL)-mediated immune response against cancer cells. CTLA4, an inhibitory receptor and member of the immunoglobulin superfamily, plays a key role in the downregulation of the immune system.