Lynn Machado, age 13, of Marion, Ohio, for her question:

WHAT IS A LYMPH NODE?

Your lymphatic system is a network of small vessels that resemble blood vessels. The system returns fluids from body tissues to the blood stream. Along the lymphatic vessels are small bean shaped accumulations called lymph nodes. They may be found at many places along the lymphatic vessels. The bumps are very small and have diameters from 1 inch to 1/25 of an inch.

The lymphatic system is necessary because fluid pressure in the body continuosly causes water, proteins and other materials to seep out of tiny blood vessels called capillaries. The fluid that has leaked out, called "interstitial fluid," bathes and nourishes body tissues.

Your tissues would become swollen if there was no way for your excess interstitial fluid to return to the blood. Most of the extra fluid seeps into capillaries that have low fluid pressure. The rest returns by way of the lymphatic system and is called lymph.

The term "node" comes from the Latin word "nodus," meaning "knot." Lymph nodes actually resemble knots in the string of lymphatic vessels. The nodes are bunched together in some areas, such as the neck, armpits and above the groin. They are also bunched in various parts of the body near a number of organs and large blood vessels.

Lymph nodes contain large cells called macrophages that absorb harmful matter and also dead tissue.

Lymph is chemically much like plasma, the liquid part of the blood. But lymph contains about half as much protein as plasma, because large protein molecules do not seep through blood vessel walls as easily as do some other substances called antibodies. Antibodies either destroy the abnormal foreign matter or make it harmless.

Lymph tissue resembles the tissue of the lymph nodes. It is found in some parts of the body that are not generally considered part of the lymphatic system. The adenoids and tonsils, as examples, are made of up lymphoid tissue as are the spleen and thymus gland.

Lymphoid tissue produces and contains lymphocytes and it aids in the body's defense against infection.

Lymphocytes are a type of white blood cell that are produced by the lymph nodes.

One of your body's most important defenses against infection comes from your lymphatic system. Harmful particles and bacteria that have entered your body are filtered out by small masses of tissue along the vast network of lymphatic vessels.

The lymphatic system is considered to be part of the circulatory system because lymph comes from the blood and returns to the blood.