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 :
Correct Answer: C) Bone marrow and thymus
Explanation:
The lymphatic system plays a crucial role in the body’s immune defense. It consists of a network of organs, tissues, and cells that work together to produce and transport lymph, a fluid containing infection-fighting white blood cells (particularly lymphocytes). Lymphoid organs are categorized into primary and secondary lymphoid organs based on their function in lymphocyte development and activation.
Primary Lymphoid Organs
These are the sites where lymphocytes (specifically B and T cells) are formed and mature:
- Bone Marrow: This is the soft tissue inside bones where hematopoiesis (formation of blood cells) occurs. It produces all blood cells, including lymphocytes. B cells mature in the bone marrow and become immunocompetent (able to recognize antigens).
- Thymus: Located behind the sternum and between the lungs, the thymus is where T cells mature. Immature T cells from the bone marrow travel to the thymus to undergo selection processes that ensure they can respond to foreign antigens but not attack the body’s own tissues.
Thus, bone marrow and thymus are the primary lymphoid organs because they are directly involved in the generation and maturation of lymphocytes.
Secondary Lymphoid Organs
These include the spleen, lymph nodes, tonsils, and appendix. They are sites where mature lymphocytes become activated in response to pathogens. Here, lymphocytes encounter antigens, proliferate, and initiate the immune response. However, they do not produce or mature lymphocytes.
Incorrect Options:
- A) Spleen and thymus: The spleen is a secondary organ.
- B) Lymph nodes and tonsils: Both are secondary.
- D) Appendix and spleen: Also secondary.
Therefore, option C (bone marrow and thymus) is the correct choice as they are the only organs responsible for lymphocyte production and maturation.