As you know, the best definition of an allergy is “a classic overreaction by the immune system to an otherwise harmless substance.” A pollen such as Bermuda grass is typically harmless, but to the atopic patient, there is the potential that his immune system will identify this pollen as harmful, setting off a cascade of events in the immune system that results in the increased production of allergen-specific IgE (in this case anti-Bermuda IgE).
Like all antibody isotypes, IgE is composed of two fragments, Fab and Fc. The Fab fragment has a high affinity to the allergen against which it was produced while the Fc fragment has a high affinity to specific receptors on the surface of mast cells. In our example, the Bermuda grass IgE bound to mast cells via the Fc fragment would bind any incoming Bermuda grass at the Fab fragment. Further exposure to the allergen results in IgE cross-linking and ultimate degranulation of the mast cell, resulting in the release of histamine, bradykinin, serine proteases and other factors that result in the onset of symptoms (see figure).
While both serum and intradermal allergy testing have their merits, this article will summarize the overall benefits of the serum testing approach. In its early years serum testing came under severe criticism, but many of these criticisms were addressed with the development of the SPOT Test. The newest iteration, the SPOT Platinum, takes serum allergy testing a stage further by making this the most sensitive and specific test available in the industry.
The goal of the test is to measure the amount of allergen specific IgE in the serum. Due to the relatively low abundance of serum IgE and interference from other antibody isotypes, namely IgA, IgM, IgD and especially IgG, standard ELISA assays are subject to dramatic false negative results. The first patented technique in the SPOT Platinum assay is binding of the allergen to the solid surface while, at the same time, retaining the three-dimensional structure of the allergen for maximum antibody binding. Concurrently, it is important to block the binding sites for all antibody isotypes other than IgE. These two steps increase both the specificity and sensitivity of the test greater