IgE or immunoglobulin E is an antibody involved in the classic allergic reactions (type I reactions). In a blood analysis one can measure serum total IgE, but can also measure specific IgE.
Specific IgE is the IgE directed against an individual allergen. For example one can measure the level of IgE against a food like peanut, or an inhalant like pollen from grasses, or house dust mites, or a substance like latex, or a medicine like penicillin.
Moreover, the allergen can have several different molecules (for example, house dust mites have molecules called Der p 1, Der p 2, Der p3, etc), and specific IgE can be measured against those isolated molecules.
An allergic person has specific IgE against the substance that causes his/her allergy. A person without allergy usually has not that specific IgE. However there are exceptions.
Some persons have that specific IgE, but can tolerate the substance (for example, they have specific IgE against peanut but are able to eat peanuts with no reaction). We say they are sensitised, but not allergic. Some persons have specific IgE and react to the substance. We say they are allergic, not just sensitised.
In general, the higher the levels of specific IgE the more intense the symptoms caused by the allergy. Again, nevertheless, there are exceptions, with persons who have low levels but have severe reactions and, conversely, persons with high levels but with mild or no reactions, as commented before. The interpretation of results of the analysis must be made considering the signs and symptoms of the individual patient.
There are several companies that have developed methods for measuring specific IgE, and sometimes this analysis can receive names like RAST, CAP, Ala-STAT, ELISA or others.
The results of the analysis are not influenced if the patient is receiving the medication usually administered in allergic diseases, such as antihistamines, inhalers, except for the use of anti-IgE monoclonal antibodies (omalizumab); this makes the interpretation difficult.
Specific IgE can also be identified in skin prick tests or intradermal tests.