Which statement about lymph nodes and spleen is true?

• A. Neither organ initiates immune responses
• B. Lymph nodes detect antigens in blood
• C. Spleen detects antigens in lymph
• D. Both are sites for initiation of immune responses

Answer: (D)

Immune responses are initiated in secondary lymphoid organs where antigen presentation to naive lymphocytes occurs. Lymph nodes monitor the lymph that drains from tissues, trapping antigens and organizing B and T cell activation in distinct zones, so they start adaptive responses to tissue-derived antigens. The spleen, on the other hand, filters the blood and uses its white pulp to detect blood-borne antigens, prompting activation of immune cells there. Thus, both organs can initiate immune responses, which makes that statement true. The other ideas don’t fit because lymph nodes primarily detect antigens in lymph, not in blood, while the spleen detects antigens in blood, not in lymph.