What exactly does Immune Privilege mean?
In: Biology
Your immune system doesn’t come with a list of stuff to attack and stuff not to attack. As your immune system develops, even before you’re born, your immune system is exposed to all the molecules and proteins and things that make up *you*. Special immune cells “remember” all those things and learn that they’re normal and should not be attacked. After the immune system is done forming, it attacks anything new, that it hasn’t been exposed to before.
Certain parts of your body are cut off from your immune system before it can learn to recognize the things there and know not to attack them. Because of that, if your immune system ever *does* get in there, it won’t know that it’s *you*. It only sees something it’s never seen before and will attack it as foreign.
The inside of your eyeballs – the vitreous humor – has almost no living tissue, it’s almost all just water and collagen. There are a few colonies of white blood cells that are self sufficient and are there to break up clumps of junk and fight off any pathogens that get in. But pathogens pretty much never get in, because your eye is sealed up very effectively. No blood flows in, since there’s no cells in there that need food and oxygen. There’s a barrier between your retina and the vitreous humor so nothing passes between them.
Testicles have immune privilege because sperm doesn’t develop until well after the immune system learns what the person looks like. The proteins that develop there would appear foreign and would be attacked, so the testicles are also blocked off with a barrier and only allows oxygen and other resources to cross, but pretty effectively blocks most pathogens. Not all of them, though, and there are a few infections (like zika and ebola) that can hide out in the testicles to avoid the immune system.
Your central nervous system is a little different. It develops alongside your immune system, so it gets recognized. But it’s too delicate to allow your immune system to run amok. One of the best tools your immune system has to stop infections is to destroy your own cells when they become infected. You can’t do that to your central nervous system, it would cause way too many problems. Instead, your central nervous system is blocked off with the blood brain barrier. It blocks almost all molecules except for small ones, which very effectively stops almost all pathogens from getting in. Of course, if pathogens *do* get in, like rabies, it’s very very bad. They can do a lot of damage before your body allows immune cells to get in and really try to get rid of the infection. When that happens, your immune system ends up doing so much damage that it’s likely to kill you anyway.
Edit: also placenta and fetuses, because they are literally foreign genes and the mother’s immune system would attack them.
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