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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?

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    Allergies have a specific definition, they are abnormal reactions of the immune system to harmless substances. Intolerances (e.g. lactose intolerance) usually don't involve the immune system. – Mad Scientist Jun 3 '11 at 5:50
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    As a side-note, kiwifruit contains quite a bit of protein, so whoever told you was wrong on both counts here. – user2466 Jun 3 '11 at 5:55
  • After reading Diogo's answer, maybe this would be best moved to biology? – Andrey Jun 26 '13 at 13:34
  • Your kiwi allergy would be a "true" allergy if it's Oral allergy syndrome. – Laurel Sep 27 '18 at 0:28
  • Something to keep in mind: Doctors have a different definition of "allergy" than laypeople generally have. In medical-speak "allergy" refers only to to the immune system going after something harmless. In lay-speak, "allergy" generally refers to an adverse reaction to something harmless and where another identified mechanism (ie, lactose intolerance) doesn't explain it. – Loren Pechtel Oct 1 '18 at 3:42
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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.


1Mayo Clinic: Nickel allergy.

2DermNet NZ: Nickel allergy.

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    My brother has a severe allergy to codeine, which is also most certainly not a protein-like substance. Most drug allergies would also fall into this category. – John Lyon Jun 3 '11 at 5:48
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    That's correct. The confusion around this idea is that the antibodies that are made in response to an allergen challenge are themselves proteins. Very often the allergen itself isn't a protein. I chose nickel as it is as inorganic, non-protein-like as possible. – user2466 Jun 3 '11 at 5:51
  • one could hold that the allergy is to the anti-bodies, which are produced erroneously in response to contact with an otherwise harmless substance, ergo a protein. But that'd be severely stretching the definition in order to support a bogus argument :) – jwenting Oct 3 '11 at 9:21
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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.

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    Assuming this is true - and I don't have the knowledge to tell - it's most of an excellent answer. What it needs is references, which are required here. If you can put in a reference to different sorts of hypersenstivities, including something about nickel being a type 4 and type 1 being "true", that would make it much better. – David Thornley Sep 26 '18 at 21:52
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    Welcome to Skeptics! Please provide some references to support your claims. – Oddthinking Sep 26 '18 at 23:01
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    This answer is correct that type 1 hypersensitivity is what we generally think of as a "true" allergy, i.e., one that can lead to anaphylaxis, but incorrect that it requires IgE cross linking. It requires mast cell degranulation, which is typically mediated by IgE, but can occur with IgE independent release. An additional technical point: antibodies do recognize non protein antigens (sugars, lipids, nucleic acids). T cell receptors don't. So you don't get T-cell help (no class switching or affinity maturation). – De Novo supports GoFundMonica Sep 27 '18 at 2:46

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