Question

Which is a primary lymphoid organ?
A. Bone marrow and spleen
B. Spleen and thymus
C. Bone marrow and tonsils
D. Thymus gland and bone marrow

Answer 1

Hint: Primary lymphoid organs are the site of formation of lymphocytes. Production of lymphocytes or B and T cells occurs in it. Bone marrow is the site of formation of B and T cells. Lymph nodes, spleen and tonsils are considered as secondary lymphoid organs.

Complete answer:
Lymph is the fluid which flows in lymphatic vessels, lymphatic tissue and red bone marrow. When the fluid filters out of capillaries and drains into lymphatic vessels to become lymph. The content of intestinal fluid and lymph fluid is the same. Lymph drains into venous blood; it drains interstitial fluid, transport dietary lipids and facilitates immune responses. There are mainly two types of lymphoid organs: primary lymphatic organs and secondary lymphoid organs.
Primary lymphatic organs are the sites of formation and maturation of lymphocytes. The red bone marrow and the thymus gland are the two primary lymphatic organs. They also provide an environment for the stem cells to divide and mature into B and T cells. Bone marrow produces both B and T cells. Maturation of B cells occurs in bone marrow while the T cells migrate to the thymus, which is where they mature in the thymus.
Secondary lymphoid organs include lymph nodes, spleen, tonsils, Peyer's patches and mucosa associated lymphoid tissue (MALT).
From the above information we have found that thymus gland and bone marrow are primary lymphoid organs.

Hence, the correct answer is option (D).

Note: Secondary lymphoid organs help in monitoring the contents of the extracellular fluids, i.e. lymph, tissue fluid and blood. Secondary lymphocytes are always found at the site of lymphocytes activation. Lymph nodes help in filtering the lymph. They are mostly found in the neck, thorax, abdomen and pelvis. B and T cells present in lymph enter into the nodes through blood stream. Macrophages are also present in the lymph fluid.