Blood tests for specific diseases look for either antibodies that the body has created to fight that disease or a combination of both antibodies and viral antigen. Blood plasma levels are normally very low because it is normally restricted to muscle tissue. Raised levels indicate muscle trauma, which causes release of myoglobin into circulation. It is not specific to cardiac muscle, and blood levels can rise following intramuscular injection, exhaustive exercise, electrical shock cardioversion, skeletal muscle disease and muscle trauma. The immune system is a collection of specialised blood cells and their protein products, plus chemical agents, all dedicated to protecting the body against foreign, invading organisms such as bacteria and viruses. There are different blood grouping systems known. Of these, the only two used in clinical practice are the ABO system (discovered in 1901) and the rhesus (Rh) system. The other systems are ignored because any reaction they cause is mild and transient.