The immune system is a complex network of cells, tissues, and organs that together help the body fight infections and other diseases. This system is made up of the skin, mucous membranes, white blood cells, and organs and tissues of the lymph system, including the thymus, spleen, tonsils, lymph nodes, lymph vessels and bone marrow. Common symptoms of problems in the immune system include fatigue, joint pain, skin rash, abdominal pain or digestive issues, fever, swollen glands, repeated infections, or headaches. Diseases of the immune system may be diagnosed and treated by an allergist, immunologist, or rheumatologist.