Which of the following are primary lymphoid organs?
spleen and thymus
lymph nodes and tonsils
bone marrow and thymus
appendix and spleen
The correct answer and explanation is :
Correct Answer: Bone marrow and thymus
Explanation:
The lymphoid system is divided into primary (central) lymphoid organs and secondary (peripheral) lymphoid organs based on their function in immune cell development and activation.
Primary Lymphoid Organs
Primary lymphoid organs are where lymphocytes (B cells and T cells) are produced and undergo maturation before entering circulation. These include:
- Bone Marrow – This is the primary site of hematopoiesis (blood cell production) and the maturation site for B cells. All blood cells, including lymphocytes, originate from hematopoietic stem cells in the bone marrow. B cells mature here before migrating to secondary lymphoid organs.
- Thymus – This is where T cells mature. Immature T cells from the bone marrow migrate to the thymus, where they undergo selection processes to ensure they can recognize pathogens while not attacking the body’s own cells.
Secondary Lymphoid Organs
Secondary lymphoid organs are where mature lymphocytes encounter antigens and initiate immune responses. These include:
- Spleen – Filters blood and removes pathogens.
- Lymph Nodes – Trap antigens from lymph and facilitate immune cell interactions.
- Tonsils – Provide localized immune defense in the respiratory tract.
- Appendix – Contains lymphoid tissue that helps with immune responses.
Why “Bone marrow and thymus” is Correct
The spleen, lymph nodes, tonsils, and appendix are secondary lymphoid organs since they do not produce or mature lymphocytes but rather facilitate immune responses. Therefore, the correct answer is bone marrow and thymus, as these are the sites of lymphocyte production and maturation.