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.  Lymph nodes may become swollen when white blood cells are actively fighting infection.  These swollen lymph nodes can be felt just under the skin of the neck, under the arms and in the groin area.  Occasionally the lymph node can become infected (adenitis) and become red and painful.

What are lymph nodes?

Lymph nodes are collections of white blood cells that are found along the lymphatic vessels.  They inspect the lymph for bacteria, viruses and other infectious agents.  When lymph nodes are active, they become larger as more white blood cells enter the node.

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 does a lymph node feel like?

When responding to an infection, a lymph node feels like a soft, rubbery "marble" under the skin, and is often found in groups.  They are found throughout the body, but are commonly felt in the neck, behind the ears, under the arms (in the arm pits), and in the crease of the groin area.  Lymph nodes are typically less than 1 cm in diameter.

How common are swollen lymph nodes?

Nearly all children have lymph nodes that can be felt under the skin of the neck to varying degrees during childhood.  Children are constantly exposed to viruses and other pathogens that challenge the immune system and cause enlargement of the lymph nodes.  This is a sign of a normally-functioning immune system.

What causes enlarged lymph nodes?

• Many viruses (such as the common cold)
• Infectious mononucleosis
• Cat-scratch disease (often causes a single large lymph node larger than 1 cm in diameter)
• Skin bacterial infections
• Tinea infections (especially tinea capitus)
• Cancer (fortunately this is uncommon)
• Tuberculosis (fortunately this is uncommon)

When should I worry about enlarged lymph nodes?

• Larger than 1 cm in diameter in the neck 
• Larger than 1.5 cm in diameter in the groin area
• Nodes that persist for several weeks
• Matted or non-mobile lymph nodes
• When other symptoms are present such as fever, night sweats, poor weight gain, or persistent cough

What happens when a lymph node becomes infected?

Bacteria can cause a lymph node to become red, swollen, painful and warm to the touch.  Sometimes an abscess may form.  This may need to be treated with antibiotics and sometimes with incision and drainage.