The original classification by Gell and Coombs is still very useful today. They classified hypersensitivity reactions as four different types.
- Type I - IgE mediated; immediate hypersensitivity reaction.
- Type II - hypersensitivity reaction takes place intravascularly.
- Type III - immune complex mediated.
- Type IV - in this form of hypersensitivity the immune response is mediated by the action of the lymphocytes rather than by antibody.
Since this classification however, two further reactions Type V and Type VI have been described.