An allergy is an abnormal or inappropriate reaction of the body's immune system , which occurs when the body comes into contact with a substance foreign to the body (the allergen), which causes internal and external irritation.
However, we are more or less resistant to allergies depending on the state of our immune system which is determined by our lifestyle (the way we eat, what we wear, the places we frequent, our family environment, our friends, our relationship with work, etc.)