Anaphylaxis to viscotoxins of mistletoe (Viscum album) extracts

Background: Extracts of mistletoe (Viscum album) are used in many countries for adjuvant cancer therapy. These extracts contain mistletoe lectins and viscotoxins that are supposed to have immunostimulating and cytotoxic effects, respectively. The treatment is usually well tolerated. Objective: To report a case of severe anaphylaxis secondary to mistletoe extract administration with demonstrable anti-IgE antibodies to mistletoe. Methods: Skin prick tests, basophil histamine release, basophil activation test, and immunoblotting were performed to characterize the pathophysiology of this reaction. Results: The patient had immediate-type skin prick test reactions to the whole commercial preparation and to its mistletoe extract component. A histamine release test and a flow cytometric basophil activation test performed with the patient's peripheral blood leukocytes by incubation with the mistletoe extract yielded a concentration-dependent histamine release and expression of the activation marker gp53 in up to 98% of anti-IgE-positive cells. Immunoblotting revealed IgE binding to 5-kDa proteins of mistletoe in the patient's serum, which corresponds to the molecular weight of viscotoxins. The results of all these tests were negative in controls. Conclusions: Until now, anaphylaxis to mistletoe extracts has been only rarely reported. In our patient, viscotoxin specific IgE evidently had induced an anaphylactic reaction.