Immune System Disorder
The immune system is a very complex and highly developed system, yet it has a very simple mission, seek and destroy invaders. When the immune system does not function properly it leaves the body open for attacks from an array of diseases. We classify these into three broad categories; autoimmunity, immunodeficiencies, and hypersensitivities. Anything that can trigger the immune response is called an antigen. An antigen can be a microbe such as a virus, or even a part of a microbe. Tissues of cells from another person also carry nonself markers and act as antigens. This explains why tissue transplants can be rejected. In abnormal situations, the immune system can mistake self for nonself and launch an attack against the body's own cells or tissues. The result is called an autoimmune disease. Some forms of arthritis and diabetes are autoimmune diseases. In other cases, the immune system responds to a seemingly harmless foreign substance such as a dust mite. The result is allergy, and this kind of antigen is called an allergen.