Anaphylaxis to methotrexate: a possible IgE-mediated mechanism.

Abstract

This patient's urticarial reaction likely resulted from ingestion of raw maple sap and is probably related to her tree pollen allergy. She clearly produces tree pollen-specific IgE, which then recognizes cross-reacting heat-labile antigenic determinants contained in the fresh fruit and nuts of the trees. This explains her clinical reactions to these foods.' 5 Because the relevant antigens are heat-labile, she has no reaction to the cooked fruit. The raw tree sap may contain the same or similar heat-labile allergens as the pollen and fruit. This would explain this patient's reaction to the sap, as well as her ability to tolerate maple syrup. The sap is extensively boiled to produce the final maple syrup product. Any heat-labile antigens originally present in the sap would be denatured during boiling, so that the syrup itself would cause no reaction. Although this patient's reaction was limited to urticaria, it was clearly systemic in nature. Assuming an underlying IgE-mediated mechanism, the possibility of full-blown anaphylaxis in an appropriately sensitized individual can readily be appreciated. Manufacture of maple syrup is not a uniquely Canadian pastime; it is also widely practiced in the northeastern United States. Ingestion of raw sap is commonly performed to estimate its sugar content; it may occur unintentionally if partially processed sap or syrup is taste-tested. Such exposures are potentially hazardous in individuals with allergy to tree pollen or nuts. These patients should therefore be advised to avoid ingesting raw maple sap.