An allergic reaction occurs when the immune system misinterprets a normally nontoxic substance, such as grass, pollen, a detergent, or a certain food, as a harmful invader. The immune system then responds to this perceived threat, called an allergen, by releasing substances called histamines. Histamines produce a wide range of bodily reactions, including respiratory and nasal congestion, increased mucus production, skin rashes and welts, and headache. In the case of an actual threat to the body, in the form of, say, a flu virus, these reactions would form an important line of defense against the invader, helping to trap it and expel it, and encouraging you to rest and recover. But during the false alarm of an allergic response, the body overreacts to a harmless agent.
Most allergens are found either in the environment or in food. (For information about allergic reactions to food, see food allergies.) Environmental allergens include pollen (reactions to pollen are often called hay fever), mold, animal dander, dust, feathers, insect venom, certain cosmetics and household products, and metals. When the environmental allergens are removed or make their seasonal disappearance, the body returns to normal. If the allergens are not removed, the immune system will continue its artificially high state of alert. In these cases, the allergic response can develop into chronic allergic rhinitis, in which the nasal passages remain persistently inflamed.
Why some people develop allergies to certain substances and others do not remains unclear. It does seem that certain allergic responses, such as hay fever, have a genetic basis. An excess accumulation of mucus in the body, which attracts and stores the irritant, also contributes to or causes allergic responses. In addition, stress and a generally depressed immune system may contribute to the severity of allergies.
