T lymphocyte white blood cell, coloured scanning electron micrograph (SEM). T lymphocytes (T cells) are white blood cells (leucocytes) that play a critical role in the human immune response against viruses, bacteria, parasites, and tumours. They can recognise foreign antigenic peptides (short protein segments) from pathogens and tumour cells via a cell surface receptor called the T-cell antigen receptor. This recognition activates the cells into their defensive role. Magnification: x15,000, when printed 15cm wide.