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
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.
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