One of the major problems associated with human circulation of both blood and lymph is the upright posture adopted by humans. Because the heart is in the upper portion of the body, blood returning to the heart from the lower extremities has to fight gravity. Lymph, too, which must eventually re-enter circulation, has to reach the upper trunk against the force of gravity. Here, the muscular system plays an invaluable role.
The importance of the muscular system in assisting the immune system by way of lymphatic flow becomes quite apparent any time an individual must stand or sit still for a long period of time. With the legs below the level of the heart, lymph must fight gravity to return to circulation. Without regular contractions of the leg muscles, however, lymph can’t return to the upper body readily, and collects in the lower legs, producing uncomfortable swelling, called edema.
Natural killer cells (NK cells) are vital to the immune system. They kill viruses and have been shown to attack and kill cancer cells in lab tests. Decreased NK activity has been associated with increased incidence of cancer as well as an increased susceptibility to colds and infections. The researchers agree that much more information is needed and relying on self-reporting of weight-loss history has limitations.