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My understanding of allergies is that they are more or less immune responses to non-pathogenic substances. The definition on MedicineNet more or less agrees with this.

While I don't have any known allergies myself, I know people who do and they have been very vocal in telling me that they literally can't take even a bit of a substance. For example, even trace amounts of peanut dust or a bite of shellfish can send them straight into anaphylaxis and a trip to the emergency room.

My question is, if allergies are nothing more than an immune response, why don't people have allergic reactions to pathogenic substances that they have developed antibodies for? For example, considering the amount of SARS-CoV-2 circulating in my area combined with widespread vaccination for it, I would expect to see non-trivial numbers of people breaking out into hives, going into anaphylaxis, etc., upon entering a room filled with airborne coronavirus, but this doesn't actually seem to be happening. When I got my COVID-19 vaccine, I was not advised that I might now be allergic to the virus and should consider allergy medication if visiting someone likely to be infected.

For example, why don't I hear things like this regularly?

Hey, before I come into your house, I just wanted to let you know that I am highly allergic to Influenza type A H1N1, Influenza type B/Victoria, SARS-CoV-2 Delta, and several strains of Epstein-Barr common in the South Pacific. If anyone here is infected with any of them, I'm not likely to get infected per se but I might go into sudden anaphylaxis if they get into my lungs. If this happens, please use the epinephrine injector in my left pocket and call 911. Thanks!

To be clear, I know that the classic signs of an upper respiratory infection (cough, runny nose, etc.) are generally caused by the body's immune response. My concern here is that these are generally much milder and have a much more delayed onset than typical allergic reactions, which are often immediate and life-threatening (rather than taking days to develop like a typical cold or flu). People who are allergic to peanuts often carry emergency epinephrine injectors and let their friends know that they have them and might need them. People who have recovered from a viral infection that is still believed to be circulating in the community don't. My question is why?

If there are viruses or other pathogenic organisms that do commonly generate allergic reactions (e.g. persons who are immune need to stay away from sick people so they don't have an allergy flare-up if some of the virus gets in them), then that's an answer.

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The TH1/TH2 model of T-cell responsiveness can be summarized as TH1 being the "antiviral/antibacterial" immune response and TH2 is the "antiparasite" immune response against worms and other multicellular parasites (though, as with most biology, lines drawn like this are always subject to exceptions). These separate pathways counteract each other to some extent, such that you can expect a TH1 suppression when TH2 is activated, and vice-versa.

The TH2 pathway is the one associated with histamine release, which is important in the pathophysiology of anaphylaxis. It's also why people can typically take antihistamines, which suppress this part of the immune response, to suppress their environmental allergy symptoms without making them susceptible to viral and bacterial illness.

Allergic reactions including anaphylaxis tend to be more associated with the TH2 side of things: the responses against multicellular pathogens. However, that doesn't mean it's impossible for viruses to also cause anaphylactic responses:

Grunewald, S. M., Hahn, C., Teufel, M., Bröcker, E. B., Wohlleben, G., Major, T., ... & Erb, K. J. (2002). Infection with influenza A virus leads to flu antigen-induced cutaneous anaphylaxis in mice. Journal of investigative dermatology, 118(4), 645-651.

Bach, M., Lim, P. P., Azok, J., Ruda Wessell, K., Desai, A. P., & Dirajlal-Fargo, S. (2021). Anaphylaxis and rhabdomyolysis: a presentation of a pediatric patient with COVID-19. Clinical Pediatrics, 60(4-5), 202-204.

I think it's also worth considering the time course of exposure to an antigen. During an infection, antigens build over time. The initial exposure dose is potentially quite small, down to a single active viral particle, but antigens accumulate over time as the virus replicates/bacteria multiply. In the lungs the immune response can contribute to ARDS. When exposed even to a very small amount of, say, peanut dust, the antigen is still presented "all at once", hence the rapid response. No matter how small the exposure, it's going to be largest at the initial exposure and not build past that.

Anaphylactic reactions do occur in response to vaccination, though they are typically rare and associated with vaccine ingredients besides the intended "active" ingredient antigen, such as eggs for influenza vaccines. The observation period after influenza or COVID-19 vaccinations, for example, are primarily to allow nurses to be present in case of one of these rare reactions.

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    Interestingly enough there are rare cases of biphasic anaphylaxis to the mRNA vaccines onlinelibrary.wiley.com/doi/10.1002/jmv.27109 I guess it's possible then 2nd phase is caused after enough spike protein is produced, but there are plenty of other alternative explanations, as these are observed with different substances as well.
    – Fizz
    Sep 8 at 18:10

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