Cytotoxic T-lymphocyte antigen-4 (CTLA-4), molecular model. The protein receptor is found on T-lymphocytes, one of the cells of the immune system. T lymphocytes recognise a specific site on the surface of pathogens or foreign objects (antigens), bind to it, and produce antibodies or cells to eliminate that antigen. Binding of CTLA-4 has an inhibitory effect on T-lymphocytes, effectively 'turning them off'.