I know that allergic reactions like this are possible because I’ve had one. I took a medication and was fine for the first five days. Then, on day six, I started breaking out in hives.
How does this happen? I have a hard time understanding how the body can have an allergic reaction at a point when the allergen probably isn’t even in the body anymore.
In: Biology
delayed allergic reactions do happen – they’re called [Type IV hypersensitivities](https://en.wikipedia.org/wiki/Type_IV_hypersensitivity?wprov=sfti1#) and that’s what you get with poison ivy or poison oak reactions. those are because they involve other parts of your immune system (not histamines, which is why benadryl doesn’t work for poison oak) that take time to get ramped up. kind of like how it takes time for you to start showing symptoms after you catch a virus. it involves actual fighter cells from your immune system getting deployed somewhere vs normal allergies where it’s controlled by antibodies just floating around in space and messing with “infected” cells as they come across them.
a lot of drug sensitivities and contact dermatitis (rashes you get from touching an irritant) are Type IV allergic reactions. what the other commenter mentioned about needing to reach a certain threshold of medication in your system to have a visible reaction could also be a factor.
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