Allergies are bodily reactions to specific irritants known as allergens. Common allergens include pet dander, pollen and dust. Allergic reactions are caused by the body’s production of histamines, which are a natural part of the immune system response; allergy symptoms result from extreme sensitivity to certain allergens, which stimulate the overproduction of histamines, causing reactions including itchy eyes, sneezing, a runny nose, itchy palate, hives or asthma-like symptoms. More serious allergic reactions can be triggered by certain foods or medications, and may include facial swelling, constriction of the airways and anaphylaxis. In extreme cases, allergic reactions can lead to death.