Background: An anaphylactic reaction is one of the alarming adverse effects of allergen immunotherapy. Although a systemic reaction can follow a biphasic course, its incidence, its outcome, and the risk factors for its development are unknown in patients treated with immunotherapy.
Objective: To record the incidence, characteristics, and outcome of immunotherapy-inflicted biphasic reactions (BRs).
Methods: All patients attending large in-hospital allergy clinics for immunotherapy were followed up prospectively. Recorded patient data included the following: demographics, diagnosis, type and phase of immunotherapy, and peak expiratory flow (PEF) before each administration of the injections. If an anaphylactic reaction occurred, medical treatment was recorded and the patient was asked to complete a 3-day diary that included symptoms and periodic measurements of PEF. A BR was defined as a late decrease in PEF of more than 20%, with or without accompanying symptoms.
Results: During 10,040 visits, 453 patients received 21,022 immunotherapy injections and 131 anaphylactic reactions occurred. Of these reactions, 11 (10%) were biphasic. Most uniphasic reactions (URs) and all BRs occurred in patients who were being treated for allergic rhinitis. A low baseline PEF or concomitant asthma was more common in patients with BRs. Other parameters were comparable between patients with URs and BRs. All BRs were mild and resolved either spontaneously or with oral antihistamines.
Conclusions: Immunotherapy-induced BRs are uncommon. They tend to be mild and might be more common in patients with a low baseline PEF or concomitant asthma. Therefore, a long observation after the initial reaction is not required.