A while ago, someone studying a health related subject (not necessarily medicine, but I forgot) claimed that "real" allergies could only be caused by protein-like substances.
I was told this after I claimed I was allergic to kiwi fruit, which I then was told wasn't possible.
Are there in medicine some subtle definitions that differentiate between allergies in that narrow sense, and, maybe, "intolerances" of a broader sense?
No, a common allergy is that to nickel.
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 response.1
and from New Zealand Dermatological Society Incorporated:
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 dermatitis).2
Nickel was named 'Allergen of the Year for 2008' by the The American Contact Dermatitis Society.
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.
