Drugs, which are effective are also bond to exert adverse effects. This is also true for mistletoe extracts. Extracts from European mistletoe (Viscum album, VAL) belong to the complementary therapeutic regimens and are used for adjuvant
cancer therapy. This study was performed to characterise immunological reactivity of patients with adverse effects during treatment with an aqueous VAL extract (
Helixor). VAL-stimulated proliferation and
cytokine release of peripheral blood mononuclear cells (PBMC) and anti-mistletoe
lectin (ML)-1 antibody production were investigated. 34 patients with proven adverse effects due to VAL therapy (group 1) and 9 patients with unproven relation (group 2) were studied and compared to 14 tumour patients treated with VAL for more than 2 years without side effects (
TTP). VAL-stimulated proliferation of PBMC of group 1 was significantly enhanced as compared to group 2 patients and
TTP. PBMC from patients with local manifestations proliferated significantly stronger than those from patients with systemic symptoms. Anti-ML-1
antibodies of the
IgE type were produced in patients with proven adverse effects but not in patients without adverse effects. Production of Th1 and Th2 specific
cytokines varied considerably, indicating that different mechanisms were involved in the induction of adverse effects. In conclusion, our study provide evidence that adverse effects towards VAL (
Helixor) are seldom and are dominated by an application site reaction suggesting the involvement of delayed type
hypersensitivity (DTH) reactions.