If allergies are basically a pure immune response – no actual pathogen, just your body mistaking something harmless for a pathogen and freaking out – why does getting hayfever have distinct symptoms that most illnesses don’t have? Why does my immune system being set off because of harmless grass seed and pet dander give me itchy eyes and hives, but when it’s set off by a real virus it doesn’t do either of those things? On its face, shouldn’t being sick feel like “all the symptoms you get during an allergic reaction + whatever symptoms are caused by the actual harm the pathogen is doing”?
In: Biology
The difference is in how your body reacts to allergens versus actual pathogens. With allergies, your immune system overreacts to harmless things like pollen or pet dander, causing symptoms like itchy eyes and hives. This is more about an exaggerated response to something that’s not really harmful. When you’re sick with a virus, your body reacts to the infection itself, which can cause different symptoms like fever or fatigue. So while both involve the immune system, the triggers and types of responses are different.
Allergens are actual physical things just like “real” pathogens. They get inside your body and so some times your body reacts to them, even if they don’t need to. Also. your body’s immune response is very localized. So your allergy symptoms are always going to be based on that location of the harmless pathogen. If the pet dandruff gets in your eye, your eyes get itchy. You breathe in plant matter, you get post nasal drip so on and so forth.
Actual illness causing pathogens do the same thing. Virus in your lungs? You cough and generate mucus. Intestinal parasite? Diarrhea, vomiting, gas, upset stomach etc. The difference is, actual illness causing pathogens have the ability to reproduce and move around. This makes them much more severe because your body detects much more of them and they stick around longer. (This is also an extreme simplification but ELI5).
The same reason that your immune system response for a cold is different than your immune system response to a bacterial infection in a wound. Different pathogens cause different immune reactions based on what your immune system thinks it needs to do to eliminate the threat.
Because they’re different immune responses.
Allergies are from Immunoglobulin E and involve Eosinophil white blood cells. These cells contain a lot of histamines, which cause the unique symptoms.
Colds/viruses invoke a lot of different cells, but they dont invoke Eosinophils. Many other types of white cells react and release interleukins. These have a wide range of effects, from recruiting more white blood cells, starting the Immunoglobulin M and Immunoglobulin G responses (G provides future immunity), causing inflammation, fever etc.
The “boredom” is true, Eosinophils are evolved to attack parasites. And in our clean world, they get bored and start getting sensitized to weird things. Interestingly, one of the treatments for severe allergies is introducing parasitic antigens.
“Shit something’s in my respiratory system better cough or sneeze it out” Compared to “shit something’s attacking my cells better increase the temperature”
The best studies have shown in flies that allergies are a “pre-immune” response. Basically your body wants to react to an infection _before_ you get sick. The sooner the better, to increase chances that once you are infected the time to cure is shorter.
They disabled the gene responsible in flies for allergies, they found no allergies, but then a small cold wiped entire populations, while others were just fine.
The mechanism is that you get protein damage, and your body investigates. It has no cameras so it just goes to all nearby things and says “IT WAS FUCKING YOU!” and then decides to build an immunity to it. Turns out air pollution tricks our immune system to thinking that something normal is actually hurting you. Like for example you get some pollen in your nose, and some car exhaust, some protein damage and the immune system says “AHA!!! Must have been the pollen!!!” and boom, allergy.
The way the body reacts is basically the immune system having an instantaneous attack to what it thinks is a bacteria, but it isn’t and the body can’t quite fight it off because the particles are too big, and outside the realm of the immune system’s ability to fight, so the immune system goes nuts and tries to get it, but alas it keeps failing. What you get is inflammation and that’s where the danger comes from. Maybe inflammation near your bronchial tubes, and boom can’t inhale!
Follow up eli5 if my immune system is so hyper vigilant that I am allergic to everything then why don’t I have a super human immune system that keeps me from getting sick all the time? I feel like my immune system reacts to everything except pathogens brought home by the kids.
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