An allergy is a hypersensitive reaction. It is caused by the immune system mounting a response to a particular substance. This only affects susceptible individuals.
Allergens, or allergy-causing substances, can be airborne (pollens, dust, smoke), infectious agents (bacteria, fungi, parasites), foods (strawberries, chocolate, eggs, wheat), contactants (poison ivy, chemicals, dyes), or physical agents (light, heat, cold).
Allergens can affect the respiratory system (Asthma or Hay Fever), or they can affect the Skin (wheals and Rashes). Allergens may also act on the gastrointestinal tract (nausea and Vomiting). Allergic reactions to substances injected into the bloodstream can cause violent and sometimes fatal reactions.
The best treatment for an allergic reaction is prevention, and the elimination of the offending substance from the sensitive person's environment but, as with Hay Fever, this is not always possible.