Extracts of mistletoe (Viscum album) are used in many countries for adjuvant cancer therapy. The extracts contain mistletoe lectins and viscotoxins supposed to have immunostimulating and cytotoxic effects, respectively. The treatment usually is well tolerated. We here report on a 59-year-old female who had received for adjuvant treatment of colon cancer injections of a mistletoe extract (Iscador® M c. Hg, Weleda, Germany) every two weeks for more than two years without any side effects. Two months prior to presentation she suddenly had developed generalized urticaria, swelling of lips and tongue, nausea, vomiting, and shortness of breath within 10 minutes after the injection.

Allergologic tests were performed to characterize the pathomechanism of this reaction.

In the patient there were immediate type skin prick test reactions to the whole commercial preparation and to its mistletoe extract component (kindly supplied by the manufacturer). A histamine release test and a flowcytometric basophil activation test performed with the patient's peripheral blood leucocytes by incubation with the mistletoe extract yielded a concentration-dependent histamine release up to 100% of total histamine and expression of the activation marker gp53 in 98% of anti-IgE positive cells. Immunoblotting revealed in the patient's serum IgE binding to 5-kD-proteins of mistletoe which corresponds to the molecular weight of viscotoxins. All these tests were negative in controls.

Anaphylaxis to mistletoe extracts has been rarely reported. In our patient evidently viscotoxin-specific IgE had induced an anaphylactic reaction.