We asked Scott Sicherer, MD, FAAAAI about your question and his response is below.
The consistent reactions (assuming there was no other possible trigger, nor ingestion of the proposed trigger food that was tolerated), and the positive testing to the products and the positive whey sIgE seems to confirm isolated allergy to whey protein. It is certainly possible that there is a dose effect explaining why milk or other milk products are tolerated but concentrated whey protein products are not. Of course, there are examples of allergic reactions attributable to specific proteins within a food. Milk allergy onset in adults is rare, which raises a question of academic interest about the means of sensitization/loss of tolerance. One wonders if he had childhood milk allergy that resolved but he had a high threshold that was triggered by these high protein products. One wonders if he had some type of non-ingestion exposure (perhaps from working with the powder and inhaling it) to increase sensitization as has been described in adults with adult onset food allergy (oat baths, wheat flour, etc.). A recent case report (Sousa MJCS et al Eur Ann Allergy Clin Immunol 2018;50:42-4) of adult onset milk allergy attributed to a bodybuilding milk protein supplement suggested that the high protein content of the supplement itself was perhaps sensitizing (the case report patient was sensitized to casein). Neither I nor colleagues I queried at the Jaffe Food Allergy Institute have seen a case exactly like this.
All my best.
Dennis K. Ledford, MD, FAAAAI