What causes allergy?

Allergy is a generalised phenomenon which must have a basic general cause. Many causative factors and different kinds of reaction may be involved in this condition; but any theory as to its underlying causation must be capable of applying to every form of allergy. Only deficient immune system function satisfies this requirement. Immunologists' definition of allergy is distorted, and their classification of immunological reactions is unduly restrictive and in one of its types not soundly based.