A T cell is a type of lymphocyte which develops in the thymus gland and plays a central role in the immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor on the cell surface. These immune cells originate as precursor cells , derived from bone marrow ,  and develop into several distinct types of T cells once they have migrated in to the thymus gland - for which these cells are named. T cell differentiation continues even after they have left the thymus. Groups of specific, differentiated T cells have an important role in controlling and shaping the immune response by providing a variety of immune-related functions. Helper T cells also use cytokine signalling to influence regulatory B cells directly, and other cell populations indirectly.
Difference between T- lymphocytes and B- lymphocytes?
Generation of lymphocytes in bone marrow and thymus - Immunobiology - NCBI Bookshelf
T cell , also called T lymphocyte , type of leukocyte white blood cell that is an essential part of the immune system. T cells are one of two primary types of lymphocytes — B cells being the second type—that determine the specificity of immune response to antigens foreign substances in the body. 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 or become memory T cells. They are then sent to peripheral tissues or circulate in the blood or lymphatic system.