CTLA-4 (Cytotoxic T-lymphocyte-associated protein 4) is a protein receptor that plays a crucial role as an immune checkpoint in regulating immune responses. It is expressed on the surface of T cells and helps regulate the immune system to prevent it from attacking healthy cells. CTLA-4 is essential for maintaining immune homeostasis and preventing autoimmune diseases. CTLA-4 inhibits T cell activation by competitively binding to B7 molecules (CD80 and CD86). It is rapidly expressed in the early stages of T cell activation and inhibits T cell activation and proliferation by suppressing co-stimulatory signals. This inhibitory function helps to prevent excessive immune responses and the development of autoimmune diseases. CTLA-4 is of significant importance in immunotherapy as inhibiting CTLA-4 can enhance immune responses and improve anti-tumor immune responses. AntiCTLA-4 antibodies have been used in the treatment of various tumors such as melanoma. A deep understanding of the CTLA-4 protein and its function aids in the development of new immunotherapy strategies to enhance treatment outcomes for diseases like cancer.