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Pretty much every organ doctors have claimed is "useless" winds up being tied to the immune system. Other than the brain, it is the most complex and least understood system of the body. Not because we haven't studied it, but because every time we do, we just keep learning more and more.
Spend some time around an immunology lab and you're bound to hear the occasional cry of "Not another fucking protein!"
Strange that dynamic system that has to adapt to a wide selection of rapidly changing pathogens under various environmental conditions would be extremely complicated.
Go figure...
It isnt useless but you can very easily survive without it. As you can your gall bladder and a few other things. When people say the appendix is "useless" what they mean now is that the functions it may provide are arguably vastly less noticeable than the danger it poses if something goes wrong.
Mine was removed 30 something years ago in 3rd grade. I did have some auto-immune stuff happen to me like hyperthyroidism and eczema, but they all manifested in my 30s.
If it were useless we would see at least some populations in the world to have lost it altogether.
If it's useless that could be in the future. Evolution works to it's own timescales