The term "allergy"originally meant "changed reactivity" and no restrictions as to the type of immunological response in the patient were imposed. It is only in recent years that "allergy" has become synonymous with Type I hypersensitivity (IgE-mediated allergy). Four different types of immune reactions (hypersensitivity) provoked by antigens have been defined, type I – IV.

 

Type I; Atopic allergy is mediated by IgE antibodies. It is an allergic reaction that occurs immediately following contact with the allergen. Common allergens are cat-, dog- and horse epithelium, tree-, grass- and weed pollen, house dust mites and molds.

 

Type II; In these immune reactions the antibodies (of IgG or IgM type) are free in the serum, while the antigen is bound to the surface of certain cells, or is a component of the cell membrane. The antigens are often small molecules, haptens. When complement is activated the cell is damaged. Drug and transfusion reactions are some examples.

 

Type III; Free circulating IgG antibodies, precipitins, react with locally introduced antigens and an antigen-antibody complex is produced. The complement system is activated with resulting tissue damage. Allergic alveolitis is an example of such an immune reaction. Causative antigens could be extremely high concentrations of molds or plant or animal antigens.

 

Type IV; Also called delayed hypersensitivity. This immune reaction differs from the other three types of allergic reactions in that it is not caused by antibodies, but by lymphocytes. It is a specific reaction between lymphocytes and tissue antigens or small molecules which, when fixed to the cell membrane, can function as an antigen, leading to tissue damage. Classic examples are the positive tuberculin reaction, contact dermatitis (e.g. caused by nickel or chrome) and rejection of transplanted tissues.