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The lymphatic system is part of the immune surveillance system. Blood contains fluid and blood cells. The fluid, which may contain suspended foreign material such as bacteria and viruses, seeps through blood vessel walls into the tissues, where it bathes the body cells and exchanges substances with them. Some of this lymph fluid is then taken up by lymphatic vessels and passed back to the heart, where it is again mixed with the blood. On its way the fluid passes through the lymph nodes, small nodular organs located throughout the body but concentrated in certain areas such as the armpits or groin. These lymph nodes' are also known as glands or nodes or lymphoid tissue. If they detect something foreign passing through them they swell up. This is called lymphadenopathy or swollen glands. Usually this is localised (for example an infected spot on the scalp will cause lymph nodes in the neck on that same side to swell up), but when it is all over the body it is called generalized lymphadenopathy. Usually this is in response to a body-wide disease such as influenza and will go away once the person has recovered, but sometimes it can persist long-term, even when there is no obvious cause of disease. This is then called persistent generalized lymphadenopathy (PGL).

PGL is often found in cases of autoimmune disease: where the body is attacking itself. These include diseases such as rheumatoid arthritis, lupus and sarcoidosis. Some forms of cancer will also cause PGL. Sometimes, despite exhaustive investigation, no cause for PGL is found. For the patient this can continue to be a source of concern, but many humans have had PGL all their lives and suffered no ill effects. In others the PGL may persist for a decade or more and then mysteriously subside. Children often have generalised lymphadenopathy of the head and neck, or even PGL, without there being a sinister cause found. At puberty this usually goes away.

Some people may have their immune system sensitised by exposure to a living exogenous irritant such as bacterial or viral infection, when then results in PGL after the organism has been cleared from the body. In some cases the sensitisation is caused by non-living exogenous irritants such as cyclic hydrocarbons (for example resinous vapours) or pesticides and herbicides.

• Lymphatic system