Which organ is primarily involved in the immune response and has multiple lymph nodes?

The spleen is a vital organ in the immune response, primarily due to its role in filtering the blood and facilitating the immune functions of lymphocytes. It acts as a reservoir for lymphocytes, which are white blood cells essential for fighting infections. The spleen not only produces these immune cells but also destroys old or damaged blood cells, thus maintaining healthy blood and immune system function.

Additionally, the spleen is strategically involved in the detection of pathogens. It samples the blood and can trigger an immune response when it identifies harmful substances, such as bacteria or viruses. It is also richly supplied with lymph nodes and lymphatic tissue, which play an integral part in the body's defense mechanisms by housing and distributing immune cells.

The other organs listed, while important for various bodily functions, do not have the same direct and organized role in the immune response or the presence of numerous lymph nodes like the spleen does. The heart is primarily responsible for circulating blood; the brain controls the nervous system and cognitive functions; and the lungs are primarily involved in gas exchange. None of these organs fulfill the primary responsibilities associated with immune responses that involve multiple lymph nodes as effectively as the spleen.