T lymphocyte and prostate cancer cell. Coloured scanning electron micrograph (SEM) of T lymphocyte cells (yellow) attached to a prostate cancer cell. T lymphocytes are a type of white blood cell that recognise a specific site (antigen) on the surface of cancer cells or pathogens and bind to it. Some T lymphocytes then signal for other immune system cells to eliminate the cell. The genetic changes that cause a cell to become cancerous lead to the presentation of tumour antigens on the cell’s surface. Magnification: x3500 when printed at 10 centimetres wide.