Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) is a protein receptor that acts as an immune checkpoint and downregulates immune responses. It is primarily expressed on regulatory T cells and competitively binds with B7 molecules (CD80 and CD86), inhibiting T cell activation and proliferation, exerting immune suppressive effects, and maintaining immune tolerance and self-tolerance. The CTLA-4 signaling pathway suppresses T cell receptor signaling, reducing cytokine production. Specific expression of CTLA-4 is crucial for maintaining immune balance, and its expression variation or functional mutations are associated with autoimmune diseases and tumor development. Therefore, CTLA-4 is an important target for immune regulation and cancer therapy. Its inhibitors are used to enhance anti-tumor immune responses, marking its crucial role in the field of immunotherapy.