The lymphatic system carries fluid lost from capillaries back to the bloodstream through lymphatic vessels and ducts. It also transports fat and plays a major role in immunity. Lymph nodes along lymphatic pathways contain lymphocytes and macrophages that fight microbes. The tonsils, spleen, and thymus are lymphatic organs - the tonsils trap pathogens in the mouth and throat, the spleen filters the blood and destroys old red blood cells, and the thymus matures T cells that respond to foreign substances.