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
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.
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