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.
When you’re allergic to something, your immune system is basically saying “this shouldn’t be here, we need to get rid of it”, and unrightfully attacks it.
In terms of medication, it can take a while for the med to become concentrated enough in your blood for your body to realize it doesn’t like it and have a reaction.
The body has two subsections to the immune system
The innate immune system, is all the stuff that floats around in the body looking for anything foreign, when it finds it, it attacks it indiscriminately. It essentially looks for markers on your cells, and attacks anything that isn’t carrying those markers, or releases chemicals that damage everything near a target.
The adaptive immune system, isn’t concerned with immediately destroying threats, it gathers components of anything the innate immune system destroys, with the end goal of generating antibodies to that specific threat.
The innate immune system is useful because it’ll destroy anything, but it’s slow. Meanwhile the adaptive immune system’s antibodies are numerous, fast and amplify the innate immune system, but they’re targeted to a specific theat.
What this means is the first time you take that drug, your body’s innate immune system attacks it, the response is slow. That attack provides your adaptive immune system the components it needs to generate antibodies to parts of that drug, which amp up the innate system’s immune response the next time your body sees it.
A delayed reaction is a bit more complex, in that they are not antibody mediated, the cells that find foreign components and present them to the antibody making cells, produce chemicals called cytokines, these cytokines essentially tell the rest of the immune system “follow me, make more immune cells and start fighting”, the problem is some of these immune cells actually damage your own cells when they’re activated, so these cytokines start a kind of cascade that takes a while to ramp up, but ends up attracting too many of these self damaging cells, which is where your delayed reaction comes from.
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