CTLA-4, or Cytotoxic T-Lymphocyte Antigen 4, is a protein receptor that functions as an immune checkpoint and is expressed on the surface of T cells. It plays a critical role in downregulating immune responses, essentially acting as a brake on the immune system. When CTLA-4 binds to its ligands, B7-1 (CD80) and B7-2 (CD86), on antigen-presenting cells, it inhibits T cell activation and proliferation.
