1 Having too much of anything
We can become sensitive to virtually anything if we have a large enough dose of it. What constitutes too much is entirely subjective; everyone's threshold is different. But whatever that level is, going over it will cause a sensitivity reaction.
People often become sensitive to foods they most love, if they overindulge in those foods. Wheat sensitivity is very common among bread lovers.
Any stimulus has a threshold which, when exceeded, leads to sensitivity. The solution is usually to avoid the object you have become sensitive to for a period of time, after which the body will have time to recover.
In our example, the bread lover needs to abstain from bread made with wheat for a period of time. After that, it's likely that he or she will be able to eat bread again, in more moderate amounts, without a sensitivity reaction.
When the body is trying to cope with more demands that it can handle, any extra demands are a real struggle for the body.
The last straw (that broke its back) was not in itself the problem for the camel: it was the weight of the rest of the load rather than the straw itself. In the same way it's not necessarily the substance itself, which the body has reacted to, that is the problem for the body, but the overload of stress the body is bearing.
3 An association connecting the allergen, or object of sensitivity, to a situation that, on an unconscious level, the person wishes to avoid
When a person wants to avoid something but is unwilling to admit to this on a conscious level, the body will often find a way to help the person avoid it.
For example, my client had resentments against her mother and didn't want to visit her, but for various reasons wouldn't admit this to herself. Her mother owned a cat, and my client developed an allergic response to cats. This gave her a genuine reason to avoid visiting her mother, while allowing her not to admit the resentments to herself.
4 A particular congenital or genetic intolerance to a particular stimulus or allergen
There are some things that the body, by a biological necessity, can only cope with in small measures, or just needs to avoid, if at all possible.
These are the rarer cases, I believe, and this shouldn't be the escape clause that stops you taking action to heal the problem.
5 A mistake of the immune system
In fact, I suspect that the body doesn't really make a mistake at all.
What happens is that an intolerance or allergy develops at a time of stress overload. By the time stress levels have moderated the body has become accustomed to producing a particular response. There's no longer any need for the body to respond in this way, but the response has become habitual.
These are the easiest allergies to heal, and some skilled NLP practitioners and many kinesiologists can do this. (Kinesiologists, by the way, are generally able accurately to identify allergies and sensitivities.)
Healing allergies and sensitivities that require identifying and resolving specific experiences from the past, or reducing general stress levels, involves a particular expertise that very few therapists are able to offer.