Illustration showing two types of T-cells (green and red spheres) binding to a cluster of three cancer cells (centre, red and grey spheres). T cells, or T lymphocytes, are components of the body's immune system. T cell receptors recognise and bind to peptides (chain of spheres) presented by class I MHC (major histocompatibility complex) molecules on the surface of the cancer cell. This activates the T cell, which releases lytic granules that penetrate and kill the cancer cell.