Quantitation of Allergens

The complexity of biologic material and the extreme sensitivity of the IgE system, which requires only nanogram amounts of allergen, have made standardization of aeroallergens most difficult. The traditional method of standardizing and preparing allergens for clinical use is to extract a known weight of defatted pollen in a specified volume of fluid. For example, 1 g in 100 mL of fluid would yield a 1% (1:100) solution. This weight per volume system still is one of the most commonly used in clinical practice. This solution can be concentrated or diluted as needed.

Another system of measurement, preferred by some allergists and extract manufacturers, is the protein-nitrogen unit (PNU). The basis of the PNU system is the fact that most allergenic moieties of pollens are proteins, and that the ratio of protein to dry weight of pollen varies from plant to plant. In this method, nitrogen is precipitated by phosphotungstic acid and measured by the micro-Kjeldahl technique. Total nitrogen is another method of standardization, but it offers no advantage and is used infrequently.

Both of these methods are used for other inhalant and food allergens, and clinicians generally must communicate in terms of these standards. Unfortunately, neither the weight per volume method nor the protein-nitrogen unit truly measures allergenic activity, because not all measured proteins and extractable components in the solution are allergenic. In addition, many complex allergens are destroyed during the harsh extraction procedure. Such problems have been circumvented through the use of biologic assays of "functional" allergen reactivity. Currently, ragweed pollen, grass pollen, house dust mite, and cat allergen extracts are standardized, and their activity is expressed in allergen units (AU) or biologic allergenic units (BAU). Other allergen concentrations may be added to this list in the future. It is essential for anyone devising immunotherapy regimens to have an appreciation for the biologic assays of allergenicity. These assays are described later.

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