Lymph nodes are part of the lymph system, which carries lymph fluid, nutrients, and waste material between the body tissues and the bloodstream. The lymph system is also an important part of the immune system, the body's defense system against disease.

The lymph nodes (sometimes called lymph glands) filter lymph fluid as it flows through them, trapping bacteria, viruses, and other foreign substances, which are then destroyed by special white blood cells called lymphocytes. Lymph nodes may be found singly or in groups; they may be as small as the head of a pin or as large as an olive. Groups of lymph nodes can be felt in the neck, groin, and underarms. Many lymph nodes in the body can't be felt.

When there is a problem in the body, the nearby lymph nodes may become swollen. For example, if a person has a throat infection, the lymph nodes in the neck may swell and become tender.