Which of the following is not a secondary lymphoid organ?

The thymus is classified as a primary lymphoid organ, which differentiates it from the secondary lymphoid organs like the spleen, lymph nodes, and tonsils. Primary lymphoid organs are involved in the maturation and development of lymphocytes; the thymus, specifically, is crucial for the maturation of T cells. In contrast, secondary lymphoid organs are the sites where immune responses are activated and where mature lymphocytes can encounter antigens. Thus, understanding the roles of these organs clarifies why the thymus does not fit into the category of secondary lymphoid organs.