Allergies occur when the body's defense system—the immune system—is hypersensitive to a harmless substance, called an allergen, and treats this substance as if it were a harmful or foreign invader. Attempting to defend itself against the allergen—a substance like pollen, dust, mold, smoke, or pet dander (scales of skin)—the body produces an antibody (a type of protein), which results in symptoms like itchy, watery eyes, a runny or stuffy nose, scratchy throat, and sneezing. In more severe allergic reactions, rashes, hives, a drop in blood pressure, difficulty breathing or even death can occur.