Do you know what the thymus does?
The thymus is a primary lymphoid tissue; along with bone marrow, it is the site where T lymphocytes, components of adaptive immunity, develop. While T cells complete maturation in the thymus, B cells complete development in the bone marrow.
Secondary lymphoid tissues, on the other hand, are sites where fully developed B and T lymphocytes circulate awaiting activation by pathogens. These include the spleen, appendix, tonsil, adenoid, Peyer’s patch, etc.