The allergic process takes place when an allergen makes contact with host tissue and is there coupled by a locally available IgE antibody. The allergen is almost always a foreign protein, although in some cases non-proteins such as antibiotics, other drugs, paints and other smaller molecules become allergenic as a result of their being carried by the host's own proteins.
Any foreign protein can serve as an allergen to an individual susceptible to forming IgE antibodies.
In order to identify to which allergenic substances an allergic subject is sensitive, one needs to identify in serum the presence of IgE antibodies that are individually specific for different allergens. A mixed bag of diagnostic tests are currently in use. In many cases, especially for severe conditions such as asthma, little or no allergy testing is done as patients are managed syptomatically using a variety of pharmaceutical agents. Skin testing involves the application of minute quantities of individual allergen solutions just beneath the skin surface. If the penetrated allergen comes in contact with matching IgE, a localized allergic reaction occurs which is marked by a localized redness and swelling at the application site.