From the American College of Allergy, Asthma and Immunology:

Allergists are specially trained to find the source of your allergy and asthma symptoms and treat them so you can feel healthy. After earning their medical degree, they must complete a three-year residency training program in either internal medicine or pediatrics. Then, allergists complete a two- to three-year fellowship in the field of asthma, allergy and immunology. Always look for a board-certified and fellowship-trained allergist to find a doctor who is an expert in treating allergies and asthma. Why? Allergies and asthma are their entire focus, and they know all the best and latest treatments to help patients get control of their symptoms.