Immunologists, physicians, and researchers who deal with the immune system and diseases of immunity generally divide allergic reactions into two major categories: immediate and delayed. Immediate reactions, also called Type I reactions, occur soon after exposure to an offending allergen, usually within one to four hours.

Immediate hypersensitivity generally involves IgE antibodies. Hay fever is a common example of this form of reaction.

Delayed responses, on the other hand, may occur as much as two to five days after exposure, sometimes even longer. Rather than antibodies, other kinds of cells, particularly T-lymphocytes, are believed to be the prime movers in this type of allergy reaction. Delayed responses are generally referred to as Type IV reactions. The poison ivy rash is a well-known example.