Allergy is a hypersensitive reaction to foreign substances called antigens that are harmless to most individuals. It affects 15-30% of the global population, many of them children. Allergies can cause symptoms like sneezing and itching and interfere with daily life. Treating allergies costs billions of dollars annually. There are four main types of allergic reactions that involve different immunoglobulins and immune cells. Common allergens come from natural sources like pollen and foods or chemical sources like drugs.