hi gresselda ! many people are allerigic to different types of substances including metals,leather,nickle etc ,me too allergic to some metals.it all depends upon individual body types and their genetic constitution.simple substances to which normal people are tolerant ,sensitive body even produce reaction to it ,because their immune system treat is as foreign and tend to generate response to it. the first time they come in contact with the substance body is already sensitized producing different types of immune cells and the second time they come in contact with same thing,body generates bigger response with all these immune cells acting against it and we happen to see it as allergy.