I know that a large part of organ transplants are if they’ll stick or get rejected, but why do they do that? I imagine if your body wants to live it’ll take what it can get so to outright refuse a new organ seems a little counterproductive unless there’s something inhibiting that.
In: Biology
> I imagine if your body wants to live
Your body is not sapient. It does not know anything other than what its DNA and proteins indicate. It does not understand the concept of “you need a liver in order to live,” because it doesn’t understand *anything at all.*
Instead, what the body sees is a lot of organic material with the wrong labels on it. Those labels are little chemical markers that bodies use to identify whether a material is “foreign” or not. Think of it like a dumb robot that can see in color really well, and knows the *exact* shade of green that identifies friendly targets from enemy targets. Enemies are ALL colors that aren’t the *exact* shade of green the robot knows. Suddenly, there’s a new target nearby, and it’s colored *spring* green when the robot is programmed to only recognize *shamrock* green. So the robot starts blasting, because the new target is not *exactly* the right color.
That’s what your immune system does. And it’s a good thing it does! Because there are lots of things (like cancer) that try to pretend to be the right color, and your immune system is *usually* able to detect and destroy them. Cancer becomes a problem when it figures out how to trick the robot, e.g. by sending a signal which turns off the “check for color” part of the robot’s code.
> I imagine if your body wants to live
Your body is great at living! That’s why it rejects organs.
Your body has an immune system, and your immune system’s entire job is recognizing foreign pathogens that can cause you harm, and destroying them. If you get an organ transplant, your immune system might think that the tissue is a foreign pathogen.
We lessen the risk of rejection two ways – one, by donor matching (which helps because if the new tissue is similar to your existing tissue, it’s less likely that your immune system will view it as a pathogen,) and two, by giving transplant patients immunosuppressive therapy – literally, ways to make their immune system less active and thus less likely to destroy the new tissue.
>unless there’s something inhibiting that
Yes, a functioning immune system.
Every moment of your life from birth to death, your immune system is killing things that want to live in and parasitise off of you. The key thing it needs to get right is to determine what is not (or no longer) you. And a transplant organ is **not you**.
There’s no higher thought involved about the body wanting to live or needing to know better than to attack a life-saving transplant. If those cells of the immune system detect cells without the right “I’m part of this body” secret handshake (HLA, human leukocyte antigen), they just kill them.
The way you get around this is by getting an organ whose secret handshake is as close a match as possible, and giving drugs that interfere with the immune system of the recipient. And it’ll still catch on eventually.
Your immune system doesn’t know it’s an organ or that you need it. Your immune system only knows to look for what is or isn’t *you*. There’s no list for what should belong in your body. As your immune system develops, mostly before you’re born, your immune system encounters a whole bunch of antigens as your body builds proteins and adds those antigens to its own list of “do not touch”.
Antigens are any molecule structure that can cause an immune response. Think of them like ID tags. Some of these ID tags occur without your body (or anything else) deliberately making them. Some of them are just the consequence of building proteins and those *are* ID tags. Sometimes, they’re deliberate – your body wants to have ID tags that are *you*, and then anything that has weird tags, it means they *aren’t* you and should be attacked.
A “matching” organ donor means that the various ID tags are *mostly* the same as what you’d find in your own body. If they don’t match, the organ gets rejected – your immune system sees ID tags that shouldn’t be in your body and attacks. But no match is perfect. Even with a match, your body just has too many unique ID tags. Someone who gets an organ transplant has to go on immunosuppressants that lower the aggression of their immune system to *delay* rejection. Rejection is inevitable, though.
HLA or Human Leukocyte Antigens in most cells make cell matching difficult in organ transplants, but being closely genetically related helps. In blood transfusion the presence of A and B antigens can complicate the process unless the recipients blood type is known. https://youtu.be/pt9ZBw8C1nk
Your immune system has an incredibly complex role because it has to detect everything in your body and decide if it is dangerous and must be neutralised, or if it’s harmless. Now unfortunately it would be impossible for your immune system to keep a catalog from birth of every single harmful invader and every harmless invader, instead it just keeps a catalog of your cells, because it knows your cells already, it’s made up of some of your cells. How it identifies your cells is proteins that are on the outside of your cells, the immune system cells can read these proteins like braille, and if it says this cell belongs to brief_skill_1487, then the immune system is happy, and if it says anything else, the immune system goes scorched earth because it isn’t capable of telling the difference between another persons kidney, and a giant colony of infectious bacteria.
Now we have some work around a to help prevent rejection. First we don’t just give kidneys out random, we have managed to develop blood tests that can test the proteins on the outside of our cells so we can compare with other peoples cells and find people who have very similar proteins on the outside of their cells (by sheer random luck, this is why finding an unrelated organ donor is a miracle and a long waiting game), and so you take a gamble and hope the immune system reads the braille and thinks it’s close enough to your cells.
Now our second strategy is by giving patients medications that weaken the immune system, like fixing the odds in your favour by making its braille reading hands have pins and needles so it’s hard to read the braille and therefore less likely to attack the new kidney. Unfortunately this also allows pathogens to cause infections easier since the immune system can’t read there proteins properly either, which is why transplant patients are considered immunocompromised
Unfortunately the new kidney doesn’t last forever and sometimes the body rejects it very fast despite it being matched and taking anti rejection meds, and even if it does work at first and works fine for quite a few years, it won’t last forever because to do that you would need to completely shut down the entire immune system, and that would allow the bacteria that live on our skin and in our mouth and in our gut to kill us VERY fast. As such over time the weakened immune system eventually starts detecting the foreign kidney and slowly attacks it until it stops working and the patient needs a new one
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