The [[bone marrow]] and [[thymus]] are primary lymphoid tissues and the sites of [[lymphocyte]] development.
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[[T cell]]s or T [[lymphocyte]]s are a type of [[lymphocyte]] (itself a type of [[white blood cell]]) that play a central role in cell-mediated [[immunity]]. They can be distinguished from other lymphocytes, such as [[B cell]]s and [[natural killer cells]] (NK cells), by the presence of a [[T-cell receptor]] (TCR) on the [[cell surface]]. They are called T cells because they mature in the [[thymus]] (although some also mature in the [[tonsil]]s).