Immune diseases

The immune system is a complex and highly developed system, yet its mission is simple: to seek and kill infectious invaders, and protect us from harmful substances. Immune system disorders occur when the immune system is either under or overactive. When the immune system is defective or underactive, people are susceptible mainly to recurrent infections (immune deficiency diseases). In severe cases, the infections are life-threatening. When the immune system is overactive, the immune system attacks our own organs causing diseases of several kinds: "autoimmune" diseases such as lupus erythematosus and rheumatoid arthritis; certain diseases of the thyroid; and certain muscle and nerve diseases, to name just a few. Allergies and asthma also are the result of an overactive immune system.