Why does the body go into anaphylactic shock if it’s potentially life threatening?
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it’s essentially an overreaction prompted by the immune system – i could be wrong, i think there’s a more complicated explanation but that’s my understanding.
The body doesn’t decide to go into anaphylactic shock. It’s an overreaction of the immune system to an antigen. What happens is massive systemic release of molecules called cytokines that normally function to recruit other immune cells, cause vasodilation, and cause capillary leakage. Under normal circumstances, these actions help direct immune cells to areas of injury/infection for your body to fight. In anaphylaxis, this is exaggerated leading to system-wide vasodilation which causes a dangerous drop in blood pressure and organ damage through lack of blood flow.
From an evolutionary standpoint, the immune response that causes allergies doesn’t benefit the individual, but all of us having an immune response capability benefits the species.
When toxins, parasites, virus or other diseases attacked a population of early humans, the percentage with the best immune response survived. Now we have some humans with overactive immune systems.
More in depth reading here: https://www.scientificamerican.com/article/dear-evolution-thanks-for/
So there are these cells called mast cells all over our body, and they’re full of histamine. If they detect foreign particles, they release the histamine. This opens up all the nearby blood vessels so that way white blood cells can come and attack the foreign particles. This allows us to crush any bacterial or viral infection very quickly. You can see this in action by giving yourself a nice hard scratch with your fingernails. Eventually the area will turn a little red.
With anaphylaxis, it’s like a little glitch with how mast cells work. Essentially, there’s this protein that is really, really good at activating mast cells and causing them release their histamine. So good that all mast cells all over the body release all their histamine. Then, all the blood vessels everywhere open up, meaning that there is more volume where blood can be in the body.
You may know from chemistry that [Initial Volume x Initial Pressure = Final Volume x Final Pressure], or more simply P1 x V1 = P2 x V2. So if the final volume is way bigger, then the final pressure needs to be way lower to keep the equation balanced.
So basically your intravascular (inside the blood vessel) volume goes way up and your blood pressure tanks so low that your heart isn’t getting enough oxygen and you die. The solution is epinephrine because it closes the blood vessels and make the heart beat harder, increasing blood pressure.
This is kind of like asking why computer software has bugs if those negatively impact the software’s function.
It’s not “intended”. It’s the result of particular elements of the immune system being tuned too trigger-happy.
E: that’s not to say this isn’t a good question. The important thing to realize is that when you get down to the cellular level, all the sensing and decision making cells do is ultimately simple chemistry — just lots of it, which makes it complicated to study. They’re really very comparable to computers in that sense.
So imagine if you had to program a computer to tell friend from foe from innocent bystander in an active warzone. If your program is too hesitant, enemies will get the drop on it, but react too aggressively and you’ll get friendly fire. Getting the tuning just right is seriously difficult.
The context we evolved in can be helpful to consider as well. Our relatively very healthy (compared to living as primitive humans) lifestyles, free of many parasites and infectious diseases, isn’t the kind of setting our immune system evolved in. Some of the issues we have with allergies as a species are thought to be because our immune systems are relatively over-active due to the comparative absence of stuff like parasites living within us that would normally modulate our immune system to an extent.
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