Which of the following is a component of the immune system?

• A. Bone Marrow
• B. Kidney
• C. Lungs
• D. Brain.

Answer: (A)

Immune system components include sites where immune cells originate and mature. Bone marrow is a primary hematopoietic organ where hematopoietic stem cells give rise to all blood cells, including the leukocytes that drive immunity. It provides the precursors for B cells and other immune cells, and even though T cells migrate to the thymus to complete maturation, their production starts here. This makes bone marrow a foundational source for the immune system’s cellular components. The kidney, lungs, and brain have important roles in immunity or host defense, but they are not the central sources that generate immune cells, so they are not considered primary components of the immune system.