Nickel allergy is one of the most common causes of allergic contact dermatitis.
If you have nickel allergy, your body
reacts to nickel and possibly to other
metals, such as cobalt and palladium.
In other words, it's mistakenly
identified nickel as something that
could harm you. Once your body has
developed a reaction to a particular
agent (allergen) — in this case,
nickel — your immune system will
always be sensitive to it. That means
anytime you come into contact with
nickel, your immune system will
respond and produce an allergic
Nickel allergy is one of the most
common causes of contact allergic
dermatitis. In affected individuals,
dermatitis (eczema) develops in places
where nickel-containing metal is
touching the skin. The most common
sites are the earlobes (from
earrings), the wrists (from a watch
strap) and the lower abdomen (from a
jeans stud); the affected areas become
intensely itchy and may become red and
blistered (acute dermatitis) or dry,
thickened and pigmented (chronic
A "true" allergy is a Type 1 IgE mediated hypersensitivity in which antibodies recognize specific antigens. Antibodies are created when an interplay between T cells and B cells occurs, notably the presentation of a protein on a Major HistoCompatability (MHC) complex, which induces the creation and release of antibodies that the target that protein. As MHC complexes only recognize proteins, a "true" allergy ONLY can happen with a protein. However, fruit has protein in it as well, as do most food items other than pure sugar or pure fat, so it is not impossible. I have many children in my clinic allergic to kiwi. However you could not be allergic to pure table sugar or pure salt, for the reason above (however pure substances dont generally exist outside of the lab and there is almost always some plant matter contaminant)
Nickle "allergy" is a delayed type 4 cell mediated hypersensitivity, in which the nickle ions act as a hapten. A hapten is a molecule that combines with a protein to form larger allergenic molecule. The ion binds with a protein and that protein is recognized by dendritic cells in the skin that cause an allergic type reaction. So it IS a protein that is being recognized, but a different type of mechanism and really not what we mean when we say a "true allergy".
Thus "true" allergies, type 1 hypersensitivities, are caused by only proteins which are omnipresent, but even exposure to non proteins can cause allergic type reactions when they combine with the bodies own proteins to form an allergenic molecule.