Allergists should also review each patient's symptoms before every injection

Allergists should also review each patient's symptoms before every injection, Nelson said. "Patients shouldn't be actively wheezing, they shouldn't be waking up at night due to asthma symptoms and their pulmonary function should be relatively normal." If symptoms are flaring up, doctors should postpone the injection.

Professional guidelines recommend that patients remain under observation for 30 to 45 minutes after an injection, so that clinicians can recognize any serious reactions and treat them immediately.

The review appears in the current issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates research in all aspects of health care. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing trials on a topic.

The meta-analysis encompasses 88 studies, with 13 published since 2001, and about 3,800 participants. All studies were randomized controlled trials. Most of the studies focus on house mite or pollen allergies, while a few looked at animal dander, mold, latex or a combination of allergens.

"Among allergists there's no question that asthma caused by allergies is responsive to immunotherapy," Nelson said. "The advantage of immunotherapy is that it causes long-lasting, if not permanent, improvement."

Furthermore, most people who have allergy-induced asthma also have nasal symptoms, Nelson said. "They have to inhale steroids into their lungs and spray them into their nose, and a lot of people don't like to do that forever. Immunotherapy treats both the nasal and chest symptoms."