Our body’s “immune privilege” and why some parts of our body have or need it and others dont.
In: Biology
Immune privilege exists where your immune system doesn’t or shouldn’t have access. There may be some immune cells there natively, but for the most part it’s walled off from the rest of it.
In the case of your brain, it’s such a delicate thing that immune cells rummaging through it could cause all sorts of problems. The better solution is to block *everything* from getting in. Between your blood vessels and your brain tissue is the blood-brain barrier that blocks nearly everything from crossing into your brain from your blood. Only small molecules like oxygen, carbon dioxide, and important hormones can cross it. But your brain does have its own immune cells and there’s evidence that it communicates with the rest of your immune system sometimes.
Your eyeball has immune privilege because, well, there’s nothing in it. Once your eye is formed and sealed, there are no living cells inside of it, only the gelatinous fluid. There’s no blood flowing into it, and no path for pathogens to get into it. And no passage for your immune cells to get used to it. Since your immune system never interacts with the inside, it never learns to recognize it as part of you. Which is fine, because it’ll never need to get in there. Your eyes do have a small number of macrophages that reproduce in there, mostly to clean up junk.
Testes have immune privilege for a similar reason. You don’t begin to produce sperm cells until after puberty, well after your immune system has learned to distinguish your cells from foreign pathogens. The problem is that sperm cells *aren’t* your cells, or at least, they don’t look like part of you. They’re only half “you”. Moreover, they are completely different kinds of cells from the rest of your body, and use parts that other cells don’t have, like a flagellum. What else has flagella? Bacteria and parasites! So your immune system doesn’t know not to attack your sperm. As a result, the testes are kept behind a barrier, like the blood-brain barrier, to protect your half-you sperms from the all you white blood cells that would attack them (and probably cause a lot of collateral damage in the process).
Women don’t have the same problem because they are, for the most part, born with all of the eggs they’ll ever have. Their immune systems interact with them and learn to recognize them. Women *do* have to deal with a fetus inside of them which is not them, which their immune system would attack. But part of the function of the uterus and the placenta is to help keep the fetus and the mother separate. The fetus has tools, too, to control the mother’s immune cells and tell them not to attack.
“Immune privilege,” simply stated, is the property of several bodily organs that means they can tolerate the introduction of foreign tissue or antigens, which the immune system has not learned to tolerate, without causing an immune response.
The common agreement seems to be that immune privilege is an adaptation so that the body runs less risk of killing or severely damaging itself in the event an immune response would have ordinarily happened.
Specifically, if the eyes or central nervous system were to experience pronounced inflammation as a result of antigen introduction, it could seriously injure us.
Another example of immune privilege is the placenta, where a baby’s blood touches its mother’s. Without immune privilege, the baby could be attacked by the mother’s immune system and cause a miscarriage.
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