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.