Antibodies are proteins that protect us against any foreign bodies or antigens, that we encounter. Antigens include proteins on viruses or toxins made by either bacterial cells or large intestinal parasites. Antibodies bind to antigens and in doing so direct the body's defence against the invading organism. Antibodies are easily detected in, and isolated from, the blood. This means that antibodies can be used to diagnose illness and to prevent or reduce disease. However it is not all good news - antibodies can also cause disease. This article reveals some of the bad effects of antibodies.