Answer;
Thymus.
After their creation, lymphocytes migrate to other immune system organs, such as the thymus, where they develop more fully and are stored until needed.
Step-by-step explanation;
The thymus is a specialized primary lymphoid organ of the immune system where the maturation of T-cells takes place.
T lymphocytes are formed from a common lymphoid progenitor in the bone marrow that also gives rise to B lymphocytes.
The progeny used to give rise to T cells leave the bone marrow and migrate to the Thymus where maturation of the T cells take place.