Allergies are hypersensitivities, overreactions of the immune system to substances that do not cause reactions in most people. Hypersensitivities are grouped into four types, I through IV. These classifications are based, to some extent, on what parts of the immune system are activated and how long it takes for a reaction to occur. The two types of hypersensitivities commonly associated with the term "allergies" are type I immediate hypersensitivities, in which antigens (allergens, foreign substances) combine with specific IgE (immunoglobulin E) antibodies to cause local and sometimes systemic reactions, usually within minutes; and type IV delayed hypersensitivities, reactions caused by the interactions of antigens with specific sensitized T lymphocytes instead of antibodies.