Allergy symptoms occur when your immune system becomes sensitized and overreacts to an allergen—something that usually is harmless, such as pollen, dust mites, molds, insect stings, or food. The same substances that trigger hay fever symptoms may also cause asthma, a condition known as allergic asthma or allergy-induced asthma.Allergies are the most common health issues affecting children in the U.S. and are the 6th leading cause of chronic illness in the U.S. affecting an estimated 40 million to 50 million people. In 2015, 8.2 percent of adults and 8.4 percent of children were diagnosed with hay fever. The chemicals released by the immune system lead to allergy signs and symptoms, such as nasal congestion, runny nose, itchy eyes, or skin reactions. For some people, this same reaction also affects the lungs and airways, leading to asthma symptoms.