T lymphocytes (or T cells) are a type of white blood cell belonging to the lymphocyte lineage, which plays an important role in the body's immune response. T lymphocytes originate from bone marrow hematopoietic stem cells and mature in the thymus. This article will discuss the development of T lymphocytes in the thymus and the classification of T lymphocytes.

Helper T cells were discovered in 1966 by a group of scientists at the University of Colorado in the United States. Helper T cells (or CD4+ T cells) are lymphoid cells originating from the bone marrow, differentiated and matured in the thymus. After maturity, they will migrate to peripheral tissues, lymph nodes or circulate in the peripheral blood.