The immune system is an exceedingly complex system of specialized cells that defend the body from infectious disease-causing organisms. Bone marrow, the lymphatic system, the thymus in the breastbone, the liver and displaying all share a major responsibility in resisting disease. Bone marrow is either red or yellow. Red marrow producers red blood cells, white blood cells and platelets. Stored in the cells of yellow marrow are lipids, which provide an important chemical energy reserve. The spleen aids the immune system by accelerating the production of red blood cells and by activating blood clotting in case of injury. Besides breaking down glycogen to provide energy, the liver can increase body temperature and induce sweating ; both of which are essential for fighting off infections. The thymus produces B cells[Plasma cells which produce antibodies] and T cells [white blood cells]. White blood cells, called lymphocytes, immobilize germs with chemical weapons. The immune system memorizes the invader that causes the disease. Once the immune system has recognized a germ and built a matching weapon, the body is able to fight off the intruder more quickly and easily next time. Many childhood diseases are set to be a kind of training ground for the immune system, helping it learn to defend itself properly and to gain strength.