An anomalous effect of allergen concentration on weal size elicited by skin testing for immediate hypersensitivity

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Abstract
Anomalous responses in the skin of atopic individuals when prick tested with serial dilutions of cocksfoot grass pollen extracts were reported. Each pollen extract was standardized using RAST [radioallergosorbent test] inhibition activity related to the WHO standard for Ig[immunoglobulin]E. Of the patients 75% gave a larger weal area with an allergen concentration of 1:105 than with the next higher concentration, i.e., 1:104. Possible mechanisms whereby a larger skin weal is elicited with a lower concentration of allergen were discussed. This is not a rare finding and can be observed in patients undergoing routine skin testing in the clinic, even where extracts are diluted over a relatively narrow concentration range.