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 organs are categorized into primary (central) lymphoid organs and secondary (peripheral) lymphoid organs based on their functions.
Primary Lymphoid Organs
Primary lymphoid organs are responsible for the production and maturation of lymphocytes (a type of white blood cell that plays a key role in the immune system). These include:
- Bone Marrow – This is the soft, spongy tissue inside bones where hematopoiesis (blood cell production) occurs. It is the site of B-cell maturation, meaning B lymphocytes develop and become immunocompetent here.
- Thymus – This is a specialized organ located behind the sternum, where T-cell maturation occurs. Immature T-cells from the bone marrow migrate to the thymus, where they undergo selection processes to become functional T-lymphocytes capable of recognizing pathogens.
Secondary Lymphoid Organs
Secondary lymphoid organs are where mature lymphocytes encounter antigens, become activated, and initiate immune responses. These include:
- Lymph nodes – Filter lymph and house immune cells that detect pathogens.
- Spleen – Filters blood and removes old red blood cells while facilitating immune responses.
- Tonsils, Peyer’s patches, appendix – Mucosa-associated lymphoid tissues (MALT) that help trap pathogens from ingested or inhaled substances.
Why Other Options Are Incorrect?
- Spleen and thymus → The spleen is a secondary lymphoid organ.
- Lymph nodes and tonsils → Both are secondary lymphoid organs.
- Appendix and spleen → Both are secondary lymphoid organs.
Thus, the correct answer is bone marrow and thymus, as they are the primary lymphoid organs responsible for lymphocyte production and maturation.