How the body’s immune system responds to harmless substances is what is referred to as allergy. Different people are allergic to different things.
An allergy is the response of the body's immune system to normally harmless substances, such as pollens, foods, and house dust mite. Whilst in most people these substances (allergens) pose no problem, in allergic individuals their immune system identifies them as a’ threat’ and produces an inappropriate response.
Allergies do not occur all at once. In short, a person cannot be exposed to a substance and become allergic to it instantly. Exposure to such a substance the first time is when the body develops protection against the invading substance.