The letter T in T-lymphocyte refers to

Thymus is haemopoietic as well as an endocrine gland. Thymus is the 'seedbed' of thymic lymphocytes (T-lymphocytes). Certain stem cells, originating in yolk sac and liver in early embryo, but only in bone marrow in late embryo, migrate into the thymus and proliferate to form a large number of lymphocytes. Thyroid is an endocrine gland. Thalamus is the part of forebrain in vertebrate lies above the hypothalamus. Tonsil is a mass of lymphoid tissue, several of which are situated at the back of the mouth and throat in higher vertebrates.