In mammals, including humans, the lymphatic system is composed of a network of thin vessels that branch, like blood vessels, into tissues throughout the body. Lymphatic vessels carry lymph, a colorless, watery fluid originating from interstitial fluid (fluid in the tissues) which is squeezed out of the blood vessels. The lymphatic system transports infection-fighting cells called lymphocytes, is involved in the removal of foreign matter and cell debris by phagocytes and is part of the body's immune system. One part of it also transports fats from the small intestine to the blood.
Lymphatic fluid gathers from the tissues and enters the valved lymphatic ducts. This 'lymph fluid' then passes back to the heart. On its way it passes through special nodular glands known as 'glands' or lymph nodes, which are concentrated in certain zones such as the back of the neck, the armpits and the groin. When a lymph node detects a possible threat passing into it in the lymph (such as a bacterium), it swells up. This is why lymph nodes swell in the region of an infected body part. Generalized lymphadenopathy (all the nodes of the body are swollen) can indicate systemic illness such infection or cancer. When generalized lymphadenopathy persists it is known as persistent generalized lymphadenopathy.