Which are main components of the immune system?

• A. Adenoid, Tonsil, Thymus, Spleen, Bone Marrow, Lymph Nodes
• B. Heart, Lungs, Liver, Kidneys
• C. Brain, Spinal Cord
• D. Adenoid, Tonsil, Thymus, Spleen, Bone Marrow, Lymph Nodes

Answer: (D)

The immune system relies on lymphoid organs that generate and organize immune cells. The list shown—adenoids, tonsils, thymus, spleen, bone marrow, and lymph nodes—includes the primary sites where immune cells mature (bone marrow and thymus) and where immune responses are coordinated and carried out (spleen, lymph nodes, and the lymphoid tissues in the throat like adenoids and tonsils). These organs work together to produce white blood cells, trap pathogens, filter bodily fluids, and mount targeted defenses.

Other body organs like the heart, lungs, liver, and kidneys play important physiological roles but are not main components of the immune system. The brain and spinal cord belong to the nervous system, not the immune system, even though immune activity can occur there in some contexts.