A lymphoid gland, the thymus is located between the lungs. Its external layer is made up of lymphatic tissue, and its internal portion contains lymphocytes. The thymus is one of the primary lymphatic organs, because lymphocytes are produced in it. The lymphocytes made here doesn't stay in the thymus, however, they move to other parts of the body and concentrate in the lymph nodes.
Although the thymus is important for T cell development in children, it begins to shrink in size as they go towards adulthood until we can't even detect it in the x-ray. T cells are educated in the thymus to recognize the foreign antigens. Without the ability to recognize self-antigens, T cells would target a person's own tissues in a very destructive way. The thymus is also responsible for fostering maturation of T cells into their various subclasses.