Which organ performs phagocytosis of abnormal cells and stores iron recycled from red blood cells and initiates immune responses by B and T cells in response to antigens in circulating blood?

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The spleen is the blood-filtering lymphoid organ that does two essential jobs: clearing abnormal circulating cells and recycling iron, and kick-starting immune responses to antigens in the blood. In the red pulp, macrophages ingest old or abnormal red blood cells and reclaim iron from hemoglobin for reuse. In the white pulp, lymphoid tissue organized around arteries—B-cell zones and T-cell zones—encounters blood-borne antigens and activates both humoral (B-cell) and cell-mediated (T-cell) immune responses. That combination—removing defective cells with iron recycling and initiating adaptive immunity against blood-borne antigens—fits the described functions best.

The liver can also perform phagocytosis and store iron via its Kupffer cells, but the explicit activation of B and T cells in response to circulating blood antigens is the hallmark of the spleen’s white pulp. The thymus mainly matures T cells and isn’t where phagocytosis of circulating abnormal cells or broad blood-borne antigen responses occur. Lymph nodes filter lymph, not circulating blood, so they aren’t the primary site for these blood-borne immune events.