The complement system is a part of the immune system that complements the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism. It is part of the innate immune system, which is not adaptable and does not change over the course of an individual's lifetime. However, it can be recruited and brought into action by the adaptive immune system. Hypersensitivity is a set of undesirable reactions produced by the normal immune system, including allergies and autoimmunity. They are usually referred to as an over- reaction of the immune system and these reactions may be damaging, uncomfortable, or occasionally fatal. Hypersensitivity reactions require a pre-sensitized (immune) state of the host. They are classified in four groups after the proposal of P. G. H. Gell and Robin Coombs in 1963.