The lymphatic system has two main functions. Its network of vessels, valves, ducts, nodes, and organs helps balance the body's fluid by draining excess fluid, known as lymph, from the body's tissue and returning it to the blood after filtering it. Some types of blood cells are also made in the lymph nodes.

The lymphatic system also plays an important role in the body's immune system. Infection, even a trivial infection is, the most common cause of swollen lymph nodes.

Let's look at a cut section of a lymph node to see what happens.

Afferent means towards. Afferent lymph vessels bring unfiltered fluids from the body into the lymph node where they are filtered.

Efferent vessels, meaning away from, carry the clean fluid away and back to the bloodstream where it helps form plasma.

When the body is invaded by foreign organisms, the swelling sometimes felt in the neck, armpits, groin, or tonsils comes from the microorganisms trapped inside the lymph nodes.

Eventually, these organisms are destroyed and eliminated by cells that line the node walls. Then the swelling and pain subside.