Which one of the given structures is not the secondary lymphoid organ?

The correct answer is Option (4) → Thymus

Lymphoid organs are where lymphocytes arise, mature and multiply. In the primary lymphoid organs (bone marrow and thymus) immature lymphocytes become antigen-sensitive. In the secondary lymphoid organs the mature lymphocytes meet the antigen and multiply.

Option (1) Spleen — A secondary lymphoid organ. It is a large bean-shaped organ that filters the blood and traps blood-borne microbes.

Option (2) Tonsils — A secondary lymphoid organ. They are lymphoid tissue in the throat where lymphocytes meet antigens entering by mouth and nose.

Option (3) Peyer’s patches — A secondary lymphoid organ. They are patches of lymphoid tissue in the small intestine.

Option (4) Thymus — Not a secondary lymphoid organ. It is a primary lymphoid organ, where T-lymphocytes mature.

Note: A similar question was also asked in CUET (UG) 2022 on 23-08-2022 (Slot 3).

NCERT Extract:
Immune System in the Body – “… Lymphoid organs: These are the organs where origin and/or maturation and proliferation of lymphocytes occur. The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes. After maturation the lymphocytes migrate to secondary lymphoid organs like spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix. The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.”