Eli5 — how do those skin prick allergy tests work? Do they show how your body metabolizes things?
In: 5
It’s a little needle that has a tiny bit of the substance in it that you could react to, pierces your skin, so your body reacts to it if you’re allergic
It introduces possible allergens into your body very locally in a way that will provoke a reaction if you are allergic (an *allergen* is a substance that could provoke an allergic reaction).
Allergies are basically your immune system attacking something it shouldn’t, causing symptoms like itching, a rash, runny nose, and so forth. What symptoms you get depend on the type of allergen and where it enters or contacts your body. Your immune cells incorrectly recognize the allergen as a threat and mount an immune response, causing the symptoms. If the allergen is in your nose, then the immune response goes there too and so you may get a runny and prickly nose. If it’s in your mouth, then you may get itchy or swollen gums, and so on.
If the allergen is introduced into your skin, then you’ll get a rash or swelling at that particular place on your skin, because the immune system only attacks there. This is useful to test for allergies because you can introduce multiple allergens at the same time in different places, and which of those spots develops a rash will tell you which things you’re allergic to.
You could also test for allergies by e.g. inhaling or ingesting the potential allergens, but this has a number of downsides: you can’t test multiple allergens at the same time this way (instead you have to do them one by one, and if you do get an allergic response you have to wait for it to subside before moving on to the next one), and also it’s likely to be more unpleasant (a small rash on your back is easier to ignore than e.g. a scratchy throat or upset stomach). Also, by doing the test on the skin, a doctor or other person can easily observe the outcome, and a tiny amount is enough to provoke a visible rash (whereas eating or inhaling a small amount may provoke only a weak reaction that will be hard for the person being tested to detect reliably).
Note that you shouldn’t do an allergy skin test if you’ve had a severe allergic reaction to anything in the past (e.g. involving anaphylactic shock), because even the tiny amounts being used in such a test could provoke another dangerous (potentially life-threatening) reaction.
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