Placebo responses in patients with house dust mite allergy

Abstract

Peripheral immune responses can be modified by behavioral conditioning. It has also been shown that the anti-allergic effects of the antihistamine desloratadine can be induced after one re-exposition to a conditioned stimulus. However, it is unknown to what extent effects of patients’ expectation contribute to this effect and whether the effect can be re-produced over time. In our study, patients with house dust mite allergy were randomly allocated to a conditioned group ( n = 25), a sham-conditioned group ( n = 25) or a natural history group ( n = 12). Patients in the conditioned group received desloratadine (unconditioned stimulus) together with a novel tasting green drink (conditioned stimulus) on five consecutive days and were re-exposed to the drink together with placebo capsules on five days after drug wash-out. Sham-conditioned patients received the green drink together with placebo capsules throughout the study, whereas natural history patients did not receive any stimuli. To analyze the (learned) placebo response, the allergic reaction was measured after the first and, to assess the reproducibility of the effect, after the fifth evocation through skin prick test and nasal provocation test. Both conditioned and sham-conditioned patients showed significantly decreased symptom scores after the first as well as after the fifth evocation, indicating that the effect is not restricted to a single event, and decreased wheal sizes after the first evocation. Our results indicate that the placebo response in allergy is probably not mainly steered by learning processes but rather by effects of expectation.