Why is the immune system so fast to respond to allergens, but take days to fight off a legitimate infection?
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The immune system is really a few different systems – the biggest distinction between an allergic response and an infectious response has to do with differences between a few of these.
One branch of your immune system works by acting very quickly against a broad set of particles it has learned to associate with infection. Its job is to try and stop an infection before it happens, or quickly stop one that has developed from getting worse by prepping the injured area for a more sustained and effective response. The signals this branch of the immune system uses do things like widen blood vessels and allow them to leak – this allows more oxygen, heat and later-responding cells to get to where the problem is faster and get out of the blood into the tissue causing trouble. It also causes redness, warmth, itching and swelling. When you have a severe allergy, it means your body has basically learned to over-use this rapid response pathway against something that isn’t actually a threat; instead of that response happening just where the problem is (such as your skin getting red and warm when you have cellulitis, or your nose being stuffy when you have a cold), it triggers the rapid response everywhere in your body, all at once – your mouth and throat swell, all your blood vessels try to dilate at once and your blood pressure tanks, and your lungs become constricted. Anaphylactic shock, in other words.
Meanwhile, there are slower systems that are used to go after threats more specifically – these are involved in allergies as well, because you need to first “learn” the antigen to make the antibodies to it, but they really shine when fighting off infection. All over your body, 24/7, there are protein machines and specialized cells that are constantly breaking down proteins they find, collecting snippets of them, and presenting them to the immune system to scan for problems. The immune system has specialized cells that scan these protein snippets (i.e. antigens) and compare them against a kind of database of every protein in the body. If they find something that’s not in the database, and there are other factors that indicate something going wrong (markers for cell damage or local inflammation, for instance), then they start building antibodies against that thing. This process takes a few days, though it’s much shorter when you have already been exposed – if that’s the case, the immune system can both discern the problem faster and has a library of cells that are already good at making antibodies against it.
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