There are two principle networks of tubes that carry fluid and blood cells through the tissues of the body.  These are blood vessels and lymphatic vessels.   When excess fluid in the body, called lymph, is not resorbed by the blood vessels, it flows into a system of vessels called lymphatics to be carried back to the bloodstream.  Along the way, lymph passes through checkpoints called lymph nodes where immune cells can inspect for germs and other pathogens.

What is lymph?

All cells in the body need water to stay alive.  Cells are like grapes that will shrivel up and become raisins if they dry out.  Blood vessels allow a nutrient-rich fluid, called lymph, to leak out and surround the cells of the body's tissues.  When the fluid becomes "dirty" it must drain through a series of vessels to be taken back to the blood stream where it can be cleaned by the blood's filtering system (which involves the liver, kidneys and spleen).  This system is similar to how the dirty water in your bath tub can be drained through the pipes of your house and eventually carried through the city's sewer system.  Once lymph returns to the bloodstream, it can be cleaned by the kidneys, spleen and liver and reused.

What are lymph nodes?

Lymph nodes are located along lymphatic vessels throughout the body.  These small pouches are packed with B cells and T cells ready to ambush any passing germ.  When an infection is detected, the cells send chemical messages throughout the body warning of an attack. Production of white blood cells in the bone marrow increases and B and T cell "warriors" are created to kill the germs.  There are about 600 lymph nodes in the body and clusters of lymph nodes are found in the neck, armpits, abdomen and groin.  Lymph nodes can swell to many times thier normal size during an infection.