Allergy specialists distinguish between different types of allergy. Different allergens trigger four mechanisms and symptoms. The reactions can occur immediately or with a delay.
Immediate-type allergy (Type I)
In this type, the symptoms occur immediately or within a few minutes. They usually affect the skin or mucous membranes. The immediate type is the most common form of allergy, as in pollen allergy, food allergy, allergy to animals or to insect venom.
Type II allergy
This is a very rare form of allergy. Blood cells may become damaged. Medicines, for example, are possible triggers of Type II allergy.
Type III allergy
This often plays a key role in drug allergy. Antibodies bind to foreign substances (antigens) that are dissolved in the blood. This gives rise to larger complexes which are deposited on the walls of the small blood vessels, e.g. in the skin or the kidneys, where they trigger an inflammatory reaction.
Delayed-type allergies (Type IV)
This is a late allergic reaction. It is the only form of allergy in which the sensitised defence cells (T-lymphocytes) act directly against allergens. In most cases, Type IV allergens are not large molecules, such as proteins, but metals or simpler chemicals. Type IV allergies appear, for instance, in the form of contact eczema, e.g. to nickel in fashion jewellery or fragrances in perfumes.