lymph vessels are part of the lymphatic system of the body.
Just like the blood circulatory system transports and delivers nutrients to all parts of the body, the lymphatic system also helps drain and return important body fluid to all parts of the body through the lymphatic system's complex network of vessels. This lymph fluid contains white blood cells, water, proteins, lipids and salts. In so doing, the lymphatic system helps defend our bodies against infection.
One of the major functions of the lymphatic system is to collect excess fluid and in particular lymph fluid which surrounds the tissues and organs of the body and return it again to the blood stream. If the lymphatic system fails to drain these excess fluids from the organs and tissues all over the body, then the lymph fluid will accumulate in the body which would then swell.