Biology · Human Health and Diseases · NEET
| Main job | Make and mature lymphocytes | Let mature lymphocytes meet antigen |
| Organs | Bone marrow, thymus (only 2) | Spleen, lymph nodes, tonsils, MALT, Peyer's patches, appendix |
| When cells arrive | Immature lymphocytes made here | Mature lymphocytes migrate here after maturation |
| Immune response | No antigen interaction here | Antigen interaction and effector-cell formation happen here |
The thymus is a PRIMARY lymphoid organ. This is a very common NEET trap. Primary lymphoid organs are only two: bone marrow and thymus. In these two, immature lymphocytes are made and become mature. The thymus is a lobed organ near the heart, under the breastbone. It is large at birth but shrinks with age, and by puberty it becomes very small. Both bone marrow and thymus give the micro-environment for T-lymphocytes to develop and mature.
The spleen is a SECONDARY lymphoid organ. NEET often puts spleen and thymus in the same option to confuse you — remember only spleen is secondary. The spleen is a large bean-shaped organ. It contains lymphocytes and phagocytes and acts as a filter of the blood by trapping blood-borne micro-organisms. It also stores a large reserve of erythrocytes (red blood cells).
Primary lymphoid organs (bone marrow and thymus) are where lymphocytes are PRODUCED and become MATURE. Secondary lymphoid organs (spleen, lymph nodes, tonsils, Peyer's patches, appendix, MALT) are where mature lymphocytes MEET the antigen. After maturing in the primary organs, lymphocytes travel to the secondary organs, interact with the antigen there, and then multiply into effector cells. So: primary = make and mature; secondary = meet antigen and respond.
Bone marrow is called the MAIN lymphoid organ because all blood cells, including lymphocytes, are produced there. Statement 'bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced' is directly from NCERT and is correct. Bone marrow is primary because it both makes lymphocytes and helps T-lymphocytes develop and mature.
Both are SECONDARY lymphoid organs. Tonsils, Peyer's patches of the small intestine, the appendix, spleen and lymph nodes are all secondary. In ReNEET 2026, tonsils and spleen were the correct secondary pair. Peyer's patches are part of the MALT (mucosa-associated lymphoid tissue) system in the gut lining.
After maturation, in primary lymphoid organs, the lymphocytes migrate for interaction with antigens to secondary lymphoid organ(s)/tissue(s) like: A. thymus, B. bone marrow, C. spleen, D. lymph nodes, E. Peyer's patches. Choose the correct answer.
Statement I: Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced. Statement II: Both bone marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes. Choose the most appropriate answer.
Which of the following are secondary lymphoid organs? (a) Bone marrow, (b) Tonsils, (c) Spleen, (d) Thymus.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
There are only two primary lymphoid organs: the bone marrow and the thymus. This is a fixed fact for NEET. All the rest are secondary.
MALT means mucosa-associated lymphoid tissue. It is lymphoid tissue in the lining of the major tracts — respiratory, digestive and urogenital. NCERT states it makes up about 50 per cent of the lymphoid tissue in the human body. MALT is a secondary lymphoid tissue.
Lymph nodes are small solid structures placed at points along the lymphatic system. They trap micro-organisms or antigens that enter the lymph and tissue fluid. The trapped antigens then activate the lymphocytes present there and start the immune response. Lymph nodes are secondary lymphoid organs.
The thymus is large at birth but keeps reducing in size as a person grows, and by puberty it becomes very small. In old age it degenerates further, so fewer thymosins are made and the immune response of older people becomes weaker. It is still a primary lymphoid organ.
Yes. The spleen contains lymphocytes and phagocytes, filters the blood by trapping blood-borne micro-organisms, and also holds a large reservoir of erythrocytes (red blood cells). It is a secondary lymphoid organ.
