The lymphatic system is an extensive drainage network that helps keep body fluid levels in balance and defends the body against infections. It is made up of a network of lymphatic vessels that carry lymph — a clear, watery fluid that contains protein molecules, salts, glucose, urea, and other substances — throughout the body.
The spleen, located in the upper left part of the abdomen under the ribcage, works as part of the lymphatic system to protect the body, clearing worn-out red blood cells and other foreign bodies from the bloodstream to help fight off infection.
One of the lymphatic system's major jobs is to collect extra lymph fluid from body tissues and return it to the blood. This is crucial because water, proteins, and other substances are always leaking out of tiny blood capillaries into the surrounding body tissues. If the lymphatic system didn't drain the excess fluid from the tissues, the lymph fluid would build up in the body's tissues, causing them to swell.
The lymphatic system also helps defend the body against germs (viruses, bacteria, and fungi) that can cause illnesses. Those germs are filtered out in the lymph nodes, which are small masses of tissue located along the network of lymph vessels. The nodes house lymphocytes, a type of white blood cell. Some of those lymphocytes make antibodies, special proteins that stop infections from spreading by trapping disease-causing germs and destroying them.
The spleen also helps the body fight infection. The spleen contains lymphocytes and another kind of white blood cell (called macrophages) that engulf and destroy bacteria, dead tissue, and foreign matter and remove them from the blood passing through the spleen.
The lymphatic system is a network of very small tubes (or vessels) that drain lymph fluid from all over the body. The major parts of the lymph tissue are located in the bone marrow, spleen, thymus gland, lymph nodes, and the tonsils. The heart, lungs, intestines, liver, and skin also contain lymphatic tissue.
One of the major lymphatic vessels is the thoracic duct, which begins near the lower part of the spine and collects lymph from the pelvis, abdomen, and lower chest. The thoracic duct runs up through the chest and empties into the blood through a large vein near the left side of the neck. The right lymphatic duct is the other major lymphatic vessel and collects lymph from the right side of the neck, chest, and arm, and empties into a large vein near the right side of the neck.
Lymph nodes are round or kidney-shaped. Most lymph nodes are about 1 cm in diameter but they can vary in size. Most of the lymph nodes are found in clusters in the neck, armpit, and groin area. Nodes are also located along the lymphatic pathways in the chest, abdomen, and pelvis, where they filter the blood. Inside the lymph nodes, lymphocytes called T-cells and B-cells help the body fight infection. Lymphatic tissue is also scattered throughout the body in different major organs and in and around the gastrointestinal tract.
The spleen helps control the amount of blood and blood cells that circulate through the body and helps destroy damaged cells.
Lymph fluid drains into lymph capillaries, which are tiny vessels. The fluid is then pushed along when a person breathes or the muscles contract. The lymph capillaries are very thin, and they have many tiny openings that allow gases, water, and nutrients to pass through to the surrounding cells, nourishing them and taking away waste products. When lymph fluid leaks through in this way it is called interstitial fluid.
Lymph vessels collect the interstitial fluid and then return it to the bloodstream by emptying it into large veins in the upper chest, near the neck.
Lymph fluid enters the lymph nodes, where macrophages fight off foreign bodies like bacteria, removing them from the bloodstream. After these substances have been filtered out, the lymph fluid leaves the lymph nodes and returns to the veins, where it re-enters the bloodstream.
When a person has an infection, germs collect in the lymph nodes. If the throat is infected, for example, the lymph nodes of the neck may swell. That's why doctors check for swollen lymph nodes (sometimes called swollen "glands" — but they're actually lymph nodes) in the neck when your throat is infected.
Certain diseases can affect the lymph nodes, the spleen, or the collections of lymphoid tissue in certain areas of the body.