Allergies are a common immune system response to substances that are usually harmless, such as pollen, dust, or certain foods. When an individual with allergies comes into contact with an allergen, their body releases histamine and other chemicals, which can cause a range of symptoms such as sneezing, itching, watery eyes, and congestion. Allergies can be seasonal, such as hay fever, or year-round, such as allergies to pet dander or dust mites. While allergies are not usually life-threatening, severe allergic reactions, called anaphylaxis, can be life-threatening and require immediate medical attention.