The lymphatic system is a system that is comprised of lymph nodes, lymph vessels, lymphoid tissues, and lymph. The system primarily functions in the collection and returning of the fluid and proteins into the blood. Sometimes, the lymphatic system is considered as part of the circulatory fluid since it works closely to the cardiovascular system. Some of the blood proteins and fluid diffuse into the lymph vessels (i.e. lymph capillaries) and they are returned into the bloodstream to maintain the normal levels of fluid and proteins in the blood. The fluid that enters the lymphatic system is called lymph. The lymph consists mainly of water, dissolved salts, and proteins. The lymph flows through lymphatic vessels and also to lymph nodes where it is filtered out of bacteria and antigens prior to entering the bloodstream. Thus, the lymphatic system is also involved in the immune defense. The lymph nodes and associated organs of lymphoid tissues (e.g. spleen, thymus, bone marrow, etc.) are sites where lymphocytes are produced. For instance, T cells involved in cell-mediated immunity are produced in the bone marrow and mature mainly in the thymus.