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Is an allergy a negative response to a molecule that the majority of the population doesn't have?

I've heard that people can be allergic to lactose, which the majority of people aren't, while people aren't considered "allergic" to poisons like bleach, which affect everyone.

Is the point of an allergy that it's unusual for people in that population to have that reaction?

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From Medicinenet.com:

A misguided reaction to foreign substances by the immune system, the body system of defense against foreign invaders, particularly pathogens (the agents of infection). The allergic reaction is misguided in that these foreign substances are usually harmless. The substances that trigger allergy are called allergen. Examples include pollens, dust mite, molds, danders, and certain foods. People prone to allergies are said to be allergic or atopic.

Poisons are not "usually harmless", therefore one can't be allergic to it, but dairy products are usually harmless and the immune system wrongly considers it a pathogen.

It's important to note that a lactose intolerance is not a lactose allergy, these are different things. An allergic reaction towards milk products is called a dairy allergy.

  • Isn't someone's allergic reaction to dairy products proof that dairy products are harmful to that person? The difference seems to be whether all or only a subset of the population has a negative reaction to the substance. – user13893 May 30 '18 at 9:16
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    No. The immune system has certain methods of detecting harmful molecules. If one is allergic, those mechanisms are broken and the immune system perceives said molecule to be harmful while in fact it isn’t. This is why treatment of anaphylactic shocks (severe reactions to allergies) is focussed on suppressing the activity of the immune system and not giving antidotes / emptying the stomach etc.(which is what you would do when there‘s poison in play) – Narusan May 30 '18 at 11:05

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