In college, Dr. Johnston was drawn to Immunology and considered a career as an immunologist. After spending time working in the lab, he realized that it would be much more exciting to treat people with immune disorders and allergy, especially since he grew up having both allergies and asthma. Dr. Johnston was attracted to Osteopathic medicine because its core principles focus on the body's ability to function as a whole and heal itself, a key role of the immune system. Dr. Johnston's clinical interests include, food allergy, eosinophilic esophagitis and hereditary angioedema (HAE), a rare genetic swelling disorder. He is involved in clinical research and serves as a principal investigator for trials aimed at treatment for eosinophilic esophagitis, hereditary angioedema and desensitization of peanut allergy.