T lymphocytes are so named because they mature in the

Explanation

Both kinds of lymphocyte originate in the bone marrow, but T cells travel to the thymus, where those that would attack the body's own antigens are eliminated and the rest acquire their receptors. B cells complete their maturation in the bone marrow itself, which supplies the other letter. The thymus is large in childhood and shrinks after puberty, once the T cell repertoire is established.