The T lymphocytes capable of killing invading cellular organisms such as bacteria, protists, or fungi are cytotoxic T cells also known as CD8+ cells. These cells are capable of recognizing whether a cell has a major histocompatibility complex I (MHC I) surface protein or not. Those with absent MHC I surface protein will be killed by the cytotoxic T cells. Those with the MHC I surface protein expressing an invading antigen will also be killed by cytotoxic T cells. Those with the MHC I surface protein without an antigen will be spared by the cytotoxic T cell.