Cytotoxic T cells to lyse cells infected with viruses. This cytotoxicity is virus specific, and only cells expressing those proteins on the surface of the infected cells are killed. As stated before, this destruction occurs only in the presence of the same MHC class I molecules. This combination directly activated CD8+cells and is a potent killer of virally infected cells. The induction of the cytotoxic T cell requires precursor cells and IL-2 from helper cells and is subject to regulation by other T cells.