White blood cells attacking cancer cells. Coloured scanning electron micrograph (SEM) of cultured adenocarcinoma cancer cells (green) being attacked by natural killer cells (NK cells, pink), a form of lymphocyte white blood cell. NK cells are able to recognise and kill abnormal cells such as tumour cells and virus-infected cells. The cancer cells are in apoptosis (programmed cell death), which is clearly visible from the blisters on their surface. After being attacked by the NK cells the cancer cells fall apart into many small 'drops' that can easily be absorbed by macrophages, another type of white blood cell. Magnification: x4500, when printed 15cm wide.