Which of the following pairs consists of the primary lymphoid organs?

The correct answer is Option (2) → Bone marrow and thymus

Lymphoid organs are where lymphocytes arise, mature and multiply. In the primary lymphoid organs, immature lymphocytes develop into antigen-sensitive lymphocytes. These are the bone marrow and the thymus. The mature lymphocytes then move to the secondary lymphoid organs, such as the spleen, lymph nodes, tonsils, Peyer’s patches and appendix, where they meet antigens.

Option (1) Bone marrow and spleen — Incorrect. Bone marrow is a primary lymphoid organ, but the spleen is a secondary lymphoid organ.

Option (2) Bone marrow and thymus — Correct. Both bone marrow and thymus are sites where immature lymphocytes mature, so this pair is made up of only the primary lymphoid organs.

Option (3) Thymus and lymph nodes — Incorrect. The thymus is a primary lymphoid organ, but lymph nodes are secondary lymphoid organs.

Option (4) Lymph nodes and Peyer's patches — Incorrect. Both are secondary lymphoid organs. Lymph nodes and the Peyer’s patches of the small intestine are where mature lymphocytes meet antigens.

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.”