What is the Lymphatic System?

The lymphatic system contains primary lymphoid organs, secondary lymphoid organs, and tertiary lymphoid tissue. The primary lymphoid organs generate white blood cells called lymphocytes. The secondary lymphoid organs maintain these cells and use them in the adaptive immune response to fight off infections, diseases, and tumors. These cells appear in the tertiary lymphoid tissue, though in smaller numbers. Typically, this tissue only assists in the immune response to fight against antigens that cause severe inflammation.

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The largest of the lymphatic organs is the spleen. During fetal development, the spleen creates red blood cells. After that, it acts as a storage unit that controls the red blood cells by removing the ones that have aged. The spleen is also responsible for the development of lymphocytes and antibodies to help fight against infections and waste. It produces these antibodies with lymphocytes that mature in the bone marrow -- B cells. These cells bind to antigens and create the antibodies to fight them.

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One of the primary lymphoid organs is the thymus. This organ possesses two lobes: the medulla and the cortex. These collect thymocyte cells from the bone marrow, which mature into T cells, lymphocytes that help the body build immunity to viruses, bacteria, and antigens. T cells can destroy infected and cancerous cells. The thymus is at its largest and most active during childhood and begins to shrink in the early teens, though T cell maturation continues at a reduced rate.

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Similar to the veins in the circulatory system, lymphatic vessels are the pathways of the lymphatic system. They consist of the lymph capillaries, the right lymphatic duct, and the thoracic duct. A clear liquid, interstitial fluid, forms in the heart and collects glucose, salt, fatty acids, healthy minerals, and waste products from the body. Around 90% of this fluid returns to the bloodstream. The lymph capillaries collect the rest of the interstitial fluid from the tissues, converting it into lymph. It is then passed through the ducts to the lymph nodes while collecting lymphocytes and proteins.

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Some of the lymphatic vessels sit at the beginning of the gastrointestinal tract, predominantly in the small intestine. These lymphatic vessels collect fats and fatty acids, enriching the fluid and turning it into chyle. This substance is important because, without fats and fatty acids, the body would lack energy and suffer from a lower temperature. These fats help protect the heart and are integral to vitamin absorption and storage.

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There are over 500 lymph nodes in the human body. They all contain a number of lymphocytes, many of which are T cells and B cells. The lymph passing through the lymph nodes carries waste materials and requires cleaning. When the lymph enters a node, it passes through white blood cells called macrophages, which trap the waste materials. The clean lymph and chyle then travel through a large vein to merge with the bloodstream.

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Several types of tonsils sit throughout the throat. These six lymphoid tissues are the body’s first line of defense against inhaled pathogens. Specialized M cells cover the surface of a tonsil. These cells can capture invading antigens and alert the B and T cells in the rest of the body to stimulate an immune response. The tonsils also produce memory B cells, which allow the body to develop an immunity to pathogens it has experienced before.

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Lymphadenopathy is a disease that affects the lymph nodes, making them abnormal in size or consistency. The most common form of lymphadenopathy is lymphadenitis, a disease that attacks the lymph nodes, causing them to swell. Generally speaking, this is typically a symptom of a minor infection such as the common cold. However, it can also be a symptom of more serious infections such as HIV/AIDS or cancer. People with this condition may experience pain or upper respiratory issues.

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Some individuals may experience a build-up of lymph called lymphedema. This fluid retention can cause severe tissue swelling and lead to ulcers and tissue-related illnesses. Individuals can develop lymphedema as an inherited issue or through damage to the lymphatic vessels. It is also related to infections in the lymphatic system, the removal of lymph nodes, and radiation therapy. The swelling may eventually make movement difficult.

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Occasionally, the lymphocytes mutate, causing lymphoma,  a blood cancer. The early symptoms of lymphoma are enlarged lymph nodes, weight loss, fatigue, and fever. Lymphoma can present as dozens of subtypes, though most hospitals classify them into Hodgkin’s lymphomas and non-Hodgkin lymphomas. Lymphoma is most likely to spread to the lungs, brain, and liver. Some subtypes are curable through radiation treatment. and chemotherapy. Treatments can help prolong survival in most other cases.

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