We have all become acquainted with a lymph node: if the family doctor has a cold and presses her fingers on the side of the neck and we wince painfully, then she has found a swollen specimen. A person has between 400 and 700 of these tissue knots, and they usually behave inconspicuously. Usually no bigger than a pea, they lie comfortably in loose fat or connective tissue and check the lymph. This slightly cloudy liquid flows through our body in so-called lymphatic channels, similar to the network of veins. The lymphatic system transports a good two liters of lymph every day, most of which is fluid that comes from the tissues. The lymph is part of the defense system, with its help, among other things, the defense cells called lymphocytes are sent through the entire body. The lymph nodes are the filter stations in the lymphatic system. A particularly large number of lymphocytes are located here and control the lymph flowing through. They filter out harmful elements and destroy them. Each lymph node has its own catchment area. Once the fluid delivered by the small lymphatic vessels has been cleaned, it returns to the blood via the larger lymphatic vessels. If a lymph node has to get rid of many unwanted invaders, for example in the event of an infection, it increases its activity and swells.