The lymphatic system is a vital part of the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more . It includes organs such as the thymus, bone marrow, spleen, tonsils, appendix, and Peyer patches in the small intestine that produce and process specialized white blood cells that fight infection and cancer.
Like the venous system, the lymphatic system transports fluids throughout the body. The lymphatic system consists of
Thin-walled lymphatic vessels
Lymph nodes
Two collecting ducts
Lymphatic vessels, located throughout the body, are larger than capillaries (the smallest blood vessels, which connect arteries and veins), and most are smaller than the smallest veins. Most of the lymphatic vessels have valves like those in veins to keep the lymph, which can clot, flowing in the one direction (toward the heart). Lymphatic vessels drain fluid called lymph from tissues throughout the body and return the fluid to the venous system through two collecting ducts.
Lymph begins as fluid that has diffused through the very thin walls of capillaries into the space between cells. Most of the fluid is reabsorbed into the capillaries and the rest is drained into the lymphatic vessels, which eventually return it to the veins. Lymph also contains many other substances including
Proteins, minerals, nutrients, and other substances, which provide nourishment to tissues
Damaged cells, cancer cells, and foreign particles (such as bacteria and viruses) that may have entered the tissue fluids
Lymph nodes are tiny bean-shaped organs that serve as collection centers for lymph. All lymph passes through strategically placed lymph nodes, which filter damaged cells, cancer cells, and foreign particles out of the lymph. Lymph nodes also contain specialized white blood cells (for example, lymphocytes Lymphocytes One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 
and macrophages Monocytes and Macrophages One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more ) designed to engulf and destroy damaged cells, cancer cells, infectious organisms, and foreign particles. Thus, important functions of the lymphatic system are to remove damaged cells from the body and to provide protection against the spread of infection and cancer. Some lymph nodes are clustered under the skin, particularly in the neck, armpits, and groin. Other lymph nodes are deep within the body, for example inside the abdomen.
The lymphatic vessels drain into collecting ducts, which empty their contents into the two subclavian veins, located under the collarbones. These veins join to form the superior vena cava, the large vein that drains blood from the upper body into the heart.
Lymphatic System: Helping Defend Against Infection
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Disorders of the lymphatic system
The lymphatic system may not carry out its function adequately due to
Blockage (obstruction): Obstruction in the lymphatic system leads to an accumulation of fluid (lymphedema Lymphedema Lymphedema is the accumulation of lymph in tissues, resulting in swelling. When lymphatic vessels are injured or obstructed, lymph fluid cannot drain and accumulates in tissues, causing swelling... read more
). Obstruction may result from scar tissue that develops when the lymph vessels or nodes are damaged or removed during surgery, by radiation therapy, by injury, or in tropical countries, by infection with a threadworm (filariasis Lymphatic Filariasis Lymphatic filariasis is infection of the lymphatic system caused by one of three species of roundworms. People have a fever, swollen lymph nodes, pain in the limbs and groin, and, if the infection... read more 
) that blocks the lymphatic ducts.Infection: Infection may cause swollen lymph nodes Swollen Lymph Nodes Lymph nodes are tiny, bean-shaped organs that filter lymph fluid. They are located throughout the body, but particular collections are found just under the skin in the neck, under the arms,... read more
because the lymph nodes are inflamed. Sometimes the lymph nodes themselves may become infected (lymphadenitis Lymphadenitis Lymphadenitis is infection of one or more lymph nodes, which usually become swollen and tender. (See also Overview of Bacterial Skin Infections.) Lymph is a fluid that oozes out of the body's... read more 
) by organisms that spread through the lymphatic system from the original site of infection.Cancer: White blood cell cancers such as lymphoma Overview of Lymphoma Lymphomas are cancers of lymphocytes, which reside in the lymphatic system and in blood-forming organs. Lymphomas are cancers of a specific type of white blood cells known as lymphocytes. These... read more
can develop in lymph nodes, and tumors in other organs may spread (metastasize) to lymph nodes near a tumor. Cancers in lymph nodes can interfere with the flow of lymphatic fluid through the node. Cancers in other areas can block lymphatic ducts. Lymphangiosarcoma is a very rare tumor that may develop in cells of the lymphatic system.
