Lymphedema is a condition where part of the body—usually an arm or a leg—becomes swollen because fluid builds up under the skin. This fluid is called lymph, and it normally travels through a network of vessels and lymph nodes (the lymphatic system) that help remove waste and fight infection. When this system doesn’t work properly, the fluid can’t drain the way it should. As a result, it collects in the tissues and causes swelling that doesn’t go away easily. What Causes Lymphedema?