Like with viruses and food poisoning.
In: Biology
Cells have to interact with each other.
To that end, the outside of them is covered with all sorts of connectors, mostly made of proteins. Think of it like puzzle pieces.
Your immune system is CONSTANTLY experimenting around with different kinds of connectors and it (mostly) builds ones that connect to things that aren’t supposed to be in your body.
So, when it runs across something that doesn’t connect, like a shard of metal, or connects to one of those immune cells specifically designed to fit puzzle pieces that aren’t supposed to be there, like a virus, it reacts. That reaction triggers other cells and a chain reaction known as an immune response begins.
Every once in awhile the immune system accidentally makes cells that fit and thus trigger on something which isn’t dangerous. We call those allergies. Sometimes it accidentally makes something that triggers on things you need, which we call auto-immune disorders.
Very ELI5, but hopefully it helps
The basic mechanism, how your cells “learn” to distinguish ”friend” vs “foe” can be simplified like this:
Your white blood cells have a “random protein generator”. By “shuffling” DNA they can create proteins that can bind to basically anything. (more https://en.wikipedia.org/wiki/V(D)J_recombination) So your bone marrows churns a lot of cells, that can bind to different stuff. If an immature white blood cell is found binding to cells in your body, it performs seppuku (apoptosis). If it can live in your bloodstream without binding to anything it matures. Now if this cell binds to something, this is recognized as enemy and the cell itself will proliferate, so that in the future the immune reaction is stronger.
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