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 :
The correct answer is: Bone marrow and thymus
Explanation:
Lymphoid organs are classified into primary and secondary lymphoid organs based on their function in the immune system.
Primary Lymphoid Organs
Primary lymphoid organs are responsible for the production and maturation of lymphocytes (a type of white blood cell essential for immunity). These organs include:
- Bone Marrow – This is the site of hematopoiesis (blood cell production) where all immune cells, including B lymphocytes (B cells), originate. B cells also mature in the bone marrow before they migrate to secondary lymphoid organs.
- Thymus – The thymus is essential for the maturation of T lymphocytes (T cells), which are critical for adaptive immunity. Immature T cells migrate from the bone marrow to the thymus, where they undergo selection processes to ensure they can recognize pathogens while avoiding attacking the body’s own cells.
Secondary Lymphoid Organs
Secondary lymphoid organs are where lymphocytes encounter antigens and initiate immune responses. These include:
- Spleen – Filters blood and helps initiate immune responses to blood-borne pathogens.
- Lymph Nodes – Filter lymphatic fluid and help initiate immune responses against pathogens in tissues.
- Tonsils, Appendix, Peyer’s Patches (GALT) – Provide immune defense against pathogens entering through mucosal surfaces.
Why Not the Other Options?
- Spleen and thymus – The spleen is a secondary lymphoid organ, so this option is incorrect.
- Lymph nodes and tonsils – Both are secondary lymphoid organs.
- Appendix and spleen – Both are also secondary lymphoid organs.
Thus, the correct answer is bone marrow and thymus, as they are responsible for lymphocyte development and maturation.