Lymph nodes are part of the lymphatic system, which carries lymph fluid, nutrients, and waste material between the body tissues and the bloodstream. The three most common sites of swollen lymph nodes are the neck, armpit, and groin. The lymphatic system is also an important part of the immune system, the body's defense system against disease.

When a lump is found in breast tissue, the nearby lymph nodes may be checked to see if cancer cells are present.