Blog Single

IgE & ALLERGY

It is common belief among General Physicians that if Total IgE is raised it is confirmatory test for Allergy. But it is serum Specific IgE ( sSIgE) and not total IgE which has significant role in diagnosis . And each antigen produces its own specific IgE.

Whenever a foreign substance or microorganism enters the human body it is usually neutralised by IgM & IgG antibody. But in case of people prone to allergy body produces IgG which is specific to particular antigen. Eg. For D. Ferinae House Dust Mite (HDM) Specific IgE to D.Ferinae mites will be produced, which after being freely circulated in blood stream will get attached to tissue mast cell.

This specific IgE which can be in minuscule amount (0.35 to 100ku/L) is usually tested by SsIgE test when it is being freely circulated in blood. SsIgE can constitute negligible amount from Total IgE which can be produced in body in huge amount in response to variety of conditions. Hence identification of elevated Total IgE as opposed to specific IgE in serum is of little diagnostic value.

The reason is that mitogenic factors in viruses (eg Cytomegalovirus, Epestein Barr virus), Bacteria (eg Staphylococcus), Helminth (eg Ascariasis) and adjuvant factors in air pollution (eg Cigarette smoke, diesel exhaust) stimulate the production of IgE molecule without initiating any allergen specific IgE sensitization.

Serum specific IgE after briefly circulating in blood gets attached to tissue mast cell and blood basophil. And when particular antigen comes in contact with it stimulate antigen-antibody reaction. This causes degranulation of mast cell leading to release of mediators of inflammation like histamine, prostaglandins, leukotrines etc and causes an allergic reaction which can be just a rash, itch, sneeze to severe form of status asthmatics and anaphylaxis.

This reaction is demonstrated by doing SPT using negligible extract of particular antigen. Hence SPT is a simple and effective method of diagnosing causative allergen. Since SsIgE detect blood levels it can be false positive as only when IgE get attached to mast cell can cause clinical reaction if it comes in contact with specific antigen. Both SPT & Specific IgE has its own Pros & Cons as crude extract is used for detecting causative antigen.

Once molecular allergy testing becomes available many of these issues of cross reactivity and others will be tackled. But as of now it is not available in India. Although Provocation Test and Food Challenge are gold standards they cannot be done in each and every case because of logistic reasons and can even stimulate a severe reaction.

Final decision of detecting the causative allergen always rests with detail clinical evaluation correlating with positive test available in our armamentarium. Although detail evaluation has to be done in a sophisticated allergy clinic the idea of this article is to know how much the general physician can use SsIgE in routine practice.

Radioallergosorbant test (RAST) by Immunocap method of detecting SsIgE is the most reliable. One should not rely on ELISA and other methods which are thrown in market. They yield non specific results and are not cost effective. The so called “Infant & Adult screening test” using secretly mixed antigen are also not recommended by any standard guidelines. So which antigens test to ask for by Immunocap method depends on detail history and patients clinical condition along with detail knowledge of local environmental condition, aerobiology of pollens etc.

For Example, if one suspects Cow’s Milk Protein Allergy SsIgE for Milk can be a reliable Test. But detail knowledge is required when you want to ask for multiple allergens. Hence it is always better to undergo Skin Prick Test and SsIgE can be done for clinical correlation if it is necessary.

No standard guidelines are recommended for Atopic Dermatitis, Allergic Rhinitis or Asthma panels and on most occasion can be misleading. One require local allergen knowledge based on aerobiology of local pollens and other common antigens based on local studies and environmental conditions if at all one needs to investigate by doing SsIgE in place of SPT. Also it is pertinent to know that these available panels contain most of the allergens relevant to western world.

For Example, respiratory allergy in geographical region with humid climate of Goa Dust Mites constitute almost 60-80% as causative antigen. Both main mites D. Ferinae & D. Pterosynnesssis can be tested. 3rd major dust mite Blomia Tropicalis which is not usually found in western world is not available for SsIgE testing. The preschool child is usually exposed to indoor allergens and along with mites, cockroach, and indoor fungus like aspergillus & alternaria, few allergenic food like milk, egg, wheat, soybeans, prawns, fish, shell fish, groundnuts and tree nuts like almonds can be tested.

School going child requires pollens to be included. In Goa grass pollens and weeds like parthenium hysterophorus and amaranthus spinosis contribute as major allergenic pollens. As per seasonal history of allergy symptoms one need to test few more tree pollens. Pet allergen can be included if history is suggestive.

In view of exorbitant cost of SsIgE (Approximate cost Rs 1500/antigen as of now) skin prick test which has more positive predictive value scores over specific IgE. In some cases of investigation of severe anaphylaxis and if anti histaminines cannot be stopped which is a prerequisite for SPT, specific IgE test can be done. Absolute eosinophilia count also can be false +ve if allergy is suspected since it can be raised in variety of manifestation from worm infestation to malignancies.

Specific Allergy Tests do have a significant role up to certain extent and can be used by physicians not trained in allergy for clinical evaluation of their patients. Final diagnosis can always be done in an allergy clinic.