T lymphocyte

T lymphocytes (also known as T-cells) are a vital component in the immune response in the body. Their name derives from their site of origin - the thymus. T-cells have receptor proteins on their cell surface membranes, these allow them to identify specific antigens displayed by other cells, e.g. phaocytes. 

There are two types of T lymphocytes: helper T-cells and cytotoxic T-cells. The role of the helper T-cells is to assist other immune system cells, whilst the cytotoxic T-cells destroy cells which have been infected, e.g. by a virus or tumour. 

https://www.immunology.org/public-information/bitesized-immunology/cells/helper-and-cytotoxic-t-cells