I know when your body is in contact with an allergen it thinks that it is a harmful "alien" but why are there different reactions person to person?

Depends on: how the body processes the "alien" and the nature of the "alien" as well. There is likely a genetic component to how one responds to "alien". How one is exposed to these "aliens" affects the respons As an example, metal allergy usually causes skin rash with minor exceptions when airborne allergens usually cause nasal , respiratory , or intestinal symptoms among genetically susceptible individuals

Dr Ali and another doctor agreed