Illustration of a cytotoxic T cell (bottom left) attacking a cancer cell (top right). T cells, or T lymphocytes, are components of the body's immune system. T cell receptors (purple) recognise and bind to peptides (chain of spheres) presented by class I MHC (major histocompatibility complex) molecules (green) on the surface of the cancer cell. This activates the T cell, which releases lytic granules (purple spheres) that penetrate and kill the cancer cell.