Allergies are an overreaction of a person’s immune system to an allergen, an otherwise harmless protein that has no effect on a non-allergic person. Common sources of inhaled allergens are tree, grass and weed pollen, mold spores, dust mites, cockroaches, cats, dogs, birds, cows, horses, rabbits and rodents. The overreaction of the immune system that results from contact with one or more of these inhaled allergens may cause annoying symptoms like coughing, sneezing, itchy eyes, runny nose, and scratchy throat. A person will often have allergies to more than one allergen.