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 the development and activation of lymphocytes.
Primary Lymphoid Organs
These are the organs where lymphocytes (B cells and T cells) are generated and mature before they migrate to secondary lymphoid organs for immune response. The primary lymphoid organs include:
- Bone Marrow – The site of hematopoiesis (formation of blood cells), including B cells and T cell precursors. B cells mature in the bone marrow before entering circulation.
- Thymus – The site where T cell precursors from the bone marrow migrate and undergo maturation. The thymus plays a crucial role in T cell differentiation and the elimination of self-reactive T cells to prevent autoimmunity.
Secondary Lymphoid Organs
These organs are responsible for trapping pathogens and activating mature lymphocytes to initiate immune responses. Examples include:
- Spleen – Filters blood, removes old red blood cells, and activates lymphocytes.
- Lymph Nodes – Filter lymph fluid and serve as sites for immune cell interactions.
- Tonsils, Appendix, Peyer’s Patches – Components of mucosa-associated lymphoid tissue (MALT) that trap pathogens from ingested or inhaled substances.
Thus, the correct answer is “Bone Marrow and Thymus”, because they are the sites of lymphocyte production and maturation, which are critical for the development of the adaptive immune system.