Which of the following are primary lymphoid organs?
A spleen and thymus
B lymph nodes and tonsils
C bone marrow and thymus
D appendix and spleen
The correct answer and explanation is :
The correct answer is C: bone marrow and thymus.
Explanation:
Lymphoid organs are classified into two categories: primary and secondary lymphoid organs. Primary lymphoid organs are where lymphocytes (a type of white blood cell) are produced and mature, while secondary lymphoid organs are sites where immune responses are initiated and lymphocytes interact with pathogens.
- Bone Marrow: Bone marrow is a primary lymphoid organ because it is the site where all blood cells, including lymphocytes (B cells and T cells), are generated. In the bone marrow, hematopoietic stem cells differentiate into various blood cell lineages, including lymphocytes. These B cells then mature in the bone marrow itself.
- Thymus: The thymus is another primary lymphoid organ. After lymphocytes (T cells) are generated in the bone marrow, they migrate to the thymus, where they mature. This maturation process involves the differentiation of T cells and the selection of those capable of distinguishing self from non-self. The thymus is essential for the development of a functional T cell repertoire, which is crucial for adaptive immunity.
Why other options are incorrect:
- A: Spleen and Thymus: While the thymus is a primary lymphoid organ, the spleen is not. The spleen is a secondary lymphoid organ that filters blood, removes old red blood cells, and provides an environment for immune cells to respond to pathogens.
- B: Lymph nodes and Tonsils: Both lymph nodes and tonsils are secondary lymphoid organs. Lymph nodes are scattered throughout the body and provide a site for immune cells to interact with pathogens, while tonsils are part of the mucosal immune system.
- D: Appendix and Spleen: Both the appendix and spleen are secondary lymphoid organs. The appendix is thought to play a role in the immune response to gastrointestinal pathogens, and the spleen filters blood for immune responses.
In summary, the primary lymphoid organs responsible for lymphocyte generation and maturation are the bone marrow and the thymus.