Why is the immune system so fast to respond to allergens, but take days to fight off a legitimate infection?
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Familiarity and dosage of the irritant. That’s how immune systems work. With people with allergies, the immune system knows what that thing is, sees a quantity that makes it notice it, and thinks that it really doesn’t like it.
Plus I doubt things like pollen and such mutate their “appearance” (so to speak). If your body identifies a specific plant pollen as something it dislikes, it’ll remember and the pollen probably didn’t really change much since last time.
With infections, you probably are getting a smaller dose and it may have changed enough that your body doesn’t see it as “the flu we had a problem with last time we saw it.” Plus infectious things are fighting back, reproducing, and doing their own actual damage to the body.
The reason you react to allergens so acutely is because you have cells sat primed with antibodies ALREADY. Your body has seen the allergen (or something sufficiently structurally similar that it can’t tell the difference) before, and it is recognised by antibody that is already sat there waiting.
Whereas most infections that actually cause symptoms are ones your body has not met before. So it has to go through the entire process of mounting an acute response: releasing ‘attack cells’ that ‘eat’ the bacteria, ‘digest’ them and then ‘present’ identifiable bits of the digested bacteria to certain other specialised cells whose job it is to take those digested pieces, and start trying to create an antibody against the bug. It can take some time to find a decent antibody – and then after a good antibody has been identified, the body then has to mass produce it. Bacteria also have ‘cunning’ methods of circumventing this system, for instance by changing the identifiable elements of themselves, hiding them away, or actively neutralising the very cells that are involved in responding.
I think a reasonable analogy is to say it’s like trying to open a lock with a key when you already know which key you need, versus having no idea and having to try every single key on the keyring. And that some bacteria are then capable of switching/hiding the locks. Plus, some peoples bodies for whatever reason struggle to even make the keys in the first place!
The allergic response is like a mine going off while the response to infection is more like a tactical operation.
The first time you’re exposed to an allergen you won’t actually have a reaction. This is just sensitisation – so the mine is primed during this phase.
The next time you encounter the allergen your primed mine will recognise it and immediately go off.
What is the mine? Mostly mast cells which “degranulate” releasing histamine which is a molecule that produces all those allergic effects.
How is it primed? Well that involves a type of immune cell called a dendritic cell encountering the allergen during the sensitisation phase, eating it up and presenting it to other immune cells called T cells. These T cell will tell other immune cells called B cells to produce antibodies called IgE.
IgE is like a Y shape, with the “prongs” able to recognise the allergen. The “trunk” is able to be embedded into the surface of the mast cell, so just imagine a mast cell with loads of IgE sticking out of it. This is now primed, or sensitised.
When the allergen comes into the body next time, it will bind to the prongs of the IgE that are sticking out of the mast cells, and that will cause the degranulation and rapid allergic reaction, like a mine going off.
As for bacterial infection, the process is similar. The dendritic cell will eat the bacteria if it encounters it, and it will present bits of it to T cells. The difference is that the T cells will recognise that this is bacteria now, and not an allergen. So instead of getting B cells to produce IgE, they get them to produce antibodies that are more suited to fighting bacteria, like IgM and IgG. These are more like bullets that are shot at bacteria directly rather than priming an intermediate like mast cells.
You are allergic to things that your immune system is already primed to fight. In a sense you have been “vaccinated” to an allergen by prior exposure, and so you mount a fast and intense immune response.
When you get sick more slowly, your immune system has encountered something that it had not learned to fight, and the reactions take a few days to ramp up as it “learns” to first detect and then react to the unfamiliar infection.
Because an allergen causes a consistent system wide PH balance change due to intake towards a reaction based on contact then saturation vs reaction and an infection usually starts at a local source that changes the PH balance that then saturates from said source of infection.
The immune system responds quickly because 1) it has memory to that antigen and 2) it has developed an IgE antibody response to the antigen. IgEs behave differently than IgG and IgM antibodies. These antibodies are very long lived and circulate in your system ready to bind their antigen. When they do they engage Fc epsilon receptors on mast cells and basophils, which leads to degranulation and release of histamine. Some times it is such a huge response that it requires an epipen to save a person’s life. BTW this is part of the innate immune response.
For a new infections your immune system creates an adaptive immune response. This takes ~7-10 days to amount a full immune response and create memory T and B cells. Once you have that memory it last a long time and sometimes for life. If you get reinfected by the same bug the memory cells usually mount an efficient response to clear it in such a way that you don’t notice and don’t have any symptoms.
Hope this helps.
Imagine you get punched in the face every day with a right hook from the same person. The force or angle may be slightly different, but in the grand scheme you’re taking the same punch that are annoying but not impactful. This is allergies.
All of a sudden, someone shows up and throws a left hook + upper-cut combo that knocks you unconscious and on your ass. This is the new illness
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