A mass of lymphoid tissue, many of which occur at intervals along the lymphatic system. Lymph in the lymphatic vessels flows through the lymph nodes, which filter out bacteria and other foreign particles, so preventing them from entering the bloodstream and causing infection. The lymph nodes also contain lymphocytes, which can multiply rapidly and mount an immune response if they encounter foreign antigens in the lymph. This leads to swelling of the lymph node. In humans, major lymph nodes occur in the neck, under the arms, and in the groin.