T- cell (also called T lymphocyte), is a type of leukocyte that is an essential part of the immune system. 'T' cells originate in the bone marrow and mature in the thymus. In the thymus, T cells multiply and differentiate into helper, regulatory, or cytotoxic T cells. They are then circulated in the blood or lymphatic system. Hence, the letter 'T' means thymus gland.