CTLA-4, or Cytotoxic T-Lymphocyte Antigen 4, is a protein found on the surface of T cells, which are crucial components of the immune system. It plays a key role in regulating immune responses by acting as an inhibitory receptor. When T cells are activated, CTLA-4 competes with another protein, CD28, for binding to the same ligands, effectively dampening the immune response to prevent overactivity. CTLA-4 is significant in cancer immunotherapy, as blocking this protein can enhance the immune system's ability to attack tumors. Drugs that inhibit CTLA-4, such as ipilimumab, have been developed to improve treatment outcomes for certain cancers, including melanoma.