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Comment by ttymck | Hacker News Reader

> Everything you're allergic to is something you were exposed to at least once; your immune system has to see it and learn to recognize it before you can react to it.

Is this true? From a biochemistry perspective, it doesn't seem like it's necessarily true. Allergies could be genetic, in which case a genetic mutation results in the phenotype where an immune system mis-identifies allergens as harmful.

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In biology anything is possible. I'd never rule out something like that. But for all common allergens I'm aware of it is true, you cannot be allergic until after your first exposure. That said, some things are "cross reactive" due to similar proteins, meaning that e.g. you could be allergic to goat's milk even though you've only been exposed to cow's milk. And some of these relationships are less obvious, like cashew and mango or banana and latex.

How do you detect that someone is allergic without exposing them? Or does the test not count? My tautology hackles are raised.

IgE tests can detect allergies without exposing the patient to the allergen. But they are imperfect. The only true test is exposure.

The test counts as a first exposure. But the reaction can only occur on a second exposure. I've often wondered if allergy tests can cause an allergy and while the allergists I've talked to don't seem to think so, I honestly don't see why not. So I would not suggest getting any unnecessary allergy tests.

If you will expose yourself to a substance, you will probably expose yourself to it hundreds of times. E.g. if you eat mussels at all, it might be something you eat now and then throughout your life. If each exposure has a certain probability of causing an allergy to the substance, then every individual exposure has a negligible risk.

Therefore, there is nothing extra risky about making the first exposure an artificial (but guaranteed to be safe) test, if you think you might be exposed in the future for any reason. In return for a negligible additional risk, you get a opportunity to discover if you are susceptible to an allergy before ever trying the substance. What am I missing here that leads to your advice of not "getting any unnecessary allergy tests"?

There are many cases that come to mind where someone has not been exposed to a potential allergen (growing up being sheltered from it, childhood aversion to it that was never reassessed in adulthood, simple lack of that allergen in previous environments) and I think it’s fair to want to avoid someone suddenly having new allergic reactions, especially when those reactions can “suck”.

My doctor says that tests can cause allergies and is therefore hesitant to give any kind of "test for everything" prick test (the ones with exposure by small cuts in your arm or back). In layman's terms as far as I've understood it, any allergic reaction, but also stress, infection, or environmental factors will get your immune system into a "alerted, defensive" state where it will learn currently present substances as things it will have to fight in the future. This can also include substances on your allergy test that are negative atm, but will be positive in the future because one currently true positive on the test aggravated your immune system.

So prick tests are a risk and you should never just blindly test, rather do the minimum necessary test for what you suspect will be positive, and only if an allergy is suspected anyways and serious enough to warrant the risk of testing.

> Is this true?

Yup! https://en.wikipedia.org/wiki/Allergy#Pathophysiology

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