ELI5, how come when you get an organ transplant, there’s a chance your body will reject it and your immune system will kill it. But when you get infected by a new disease, it’s difficult for your immune system to kill the disease.
In: Biology
When you get an organ transplant, your immune system can try to attack the organ, recognizing that it does not belong in your body. The same is true for infections. The difference is that an organ does not fight back, it does not try to spread and create more of itself.
It’s like the difference between getting into a bar fight (an infection) and going into a nursing home and beating up old people (rejection)
The virus/bacteria responsible for the disease have evolved for millions of years to survive and reproduce faster than the immune system can eliminate them.
The organ has evolved for millions of year to work with an immune system defending and not attacking it.
And the immune system has spent millions of years to attack anything that’s not supposed to be inside the body.
Diseases don’t attack us by accident. They have ADAPTED specifically to outmaneuver our immune response.
No other human has evolved anything like this. Why would it?
But your body goes WTFBBQ FIRE AT WILL!
There is a lot of sickness related damage that is actually caused by the inmune system unable to attack just what it needs to be attacked.
Okay like you’re five. With a new disease, your immune system doesn’t know it’s there relying on bumping into the virus (or what have you) by chance. With a whole organ the tissue will of course be immediately found by immune cells.
The body will always notice a transplanted organ as being from somewhere else and the body does two things with foreign objects. It’ll send in the immune system to attack and destroy, but it’ll also try to surround and barricade the object so it doesn’t interact with the body anymore. The reason transplanted organs aren’t always rejected is because the recipients take drugs to help prevent rejection. When people say “there’s a chance” they’re implying with regular usage of anti-rejection medication.
For new infections, the body will attack if it notices the infection. Not every infection behaves the same. Some infections look like part of your body so your immune system doesn’t know to attack; think penguins walking with nuns. Some viruses hide inside cells to force them to replicate so your body doesn’t see the infection until it’s overrun; this is like a trojan horse attack. Some infections are too strong and go full punisher on your body.
It’s the difference between looking out the peephole of your door and deciding not to let a stranger in, vs a group of people quietly breaking in and by the time you discover them they’ve made themselves at home and you need to chase them all out on your own with a broom.
Because the diseases that are easily killed by your immune system don’t spread, they get killed. The ones that have some mechanism that keeps them from being killed easily (usually multiple mechanisms, like stealth, or speed of reproduction, or ability to fight off the immune system) are the ones that survive and spread enough to actually succeed.
Imagine if you rolled a hundred objects of different sizes and shapes down a hill of grass. Your question is asking why the 5 that made it to the bottom weren’t stopped by all the grass. The answer is that the grass did stop 95 of them, these 5 are just the size/shape that enabled them to roll on anyways.
The fact is, your immune system is exposed to a gazillion possible infections every day. Many of these generally don’t cause illness at all because your immune system as is can do an effective job of wiping them out. Some of them tend to cause illness in people who have a weakened immune system in some way (often called opportunistic infections).
For example, your mouth and gut (or if you are a female, your lady machinery) or even your skin have tons of bacteria on them that aren’t causing serious harm and sometimes help keep harmful bacteria from taking over.
Only a few of these bacteria or viruses or whatnot will make you sick. That’s because these pathogens have evolved the ability to infect you and evade your immune system–at least for a period of time, if not permanently.
These are often somewhat species specific because they have particular adaptations that give them the capabilities to infect specific hosts. A plant virus won’t be very effective at attacking human cells. Tuberculosis probably is not very good at attacking a goldfish.
These ‘new’ diseases are ones that often have infected other animal hosts. Sometimes birds, sometimes other mammals like bats or pigs or whatnot. They can later spread to humans because:
1) they have long had the ability to infect humans but humans were hanging out in their cities and not in the middle of the forest
2)random genetic mutations gave them the ability to infect humans
3) an animal was infected with two pathogens which shared some of their genetic material creating a new strain (as often happens with the flu in pigs and birds and humans)
New pathogens which can infect humans can be extremely deadly because no one has any immunity to them since no one had the chance to be exposed and develop an immune response (think Native Americans or coronavirus).
Another reason is there has been no natural selection to temper the harm of the pathogen. Just like any living thing, natural selection favors organisms than are better able to reproduce. This tends to mean pathogens that can easily be transmissible and not kill their hosts off immediately so they can spread it to more people. Less virulent strains can take over the more deadly strains over time. Over longer periods of time, natural selection can also select for humans which are more able to fight off that pathogen. It is a never ending arms race between infectious agents and your immune system.
When it comes to organ transplants, there is a variety of types of rejection. But in general, everyone has immune responses to human tissues that are NOT from your own body. This is why blood types have to be matched for a transfusion. Transplanted organs also have blood vessels which are not yours and can be attacked in a similar way. The cells themselves also readily suggest to the immune system that something is ‘wrong’ with them triggering an immune response. For this reason, organ transplants also have to be matched carefully to find people with compatible organs.
Cancer arises when abnormal cells have not only developed the ability to grow out of control and invade other parts of your body, they also develop traits that allow them to escape the immune response that would normally wipe them out.
Your organs don’t try to hide their identity. They proclaim it loudly to anything that runs into them. There’s normally no reason for your organs to hide their identity.
All the cells in my liver have big, obvious ID badges that say “I AM /U/OPTRODE’S LIVER”. There is normally a good thing. If my liver (or part of it, more likely) got transplanted into another person, my liver cells are going to carry on loudly yelling “I AM /U/OPTRODE’S LIVER”, and that’s going to attract attention from the recipient’s immune system, because under 99.99999% of circumstances, something in one of your organs that isn’t part of you is a bad thing.
Viruses and bacteria, on the other hand, have evolved to be hard to detect. They don’t wear any obvious ID badges, since their whole business model involves being in places where if they get recognized they’ll be killed on sight.
Latest Answers