In some cases hearing of unextraordinary events helps to calibrate what extraordinary is. I have never suffered a spleen injury, but have heard of lots of young men having theirs removed when I was younger. This anecdote leads me to believe those may have been extraordinary cases. At the very least I no longer think doctors routinely just remove spleens as I once did.
Spleens are essentially large bags of blood and immunological pulp, and the reason they evolved this way is because they act as a filter against bloodborne pathogens. They host a diverse but specialized collection of immune cells, such as macrophages, and have a truly unique and interesting role in overall health.
You should read up on the thymus next. That one is super important and is also implicated in aging.
That one seems to self-deactivate with age.
It's unfortunate, because our responses to immune challenges - including cancer - diminish with age.
Anyway, I got to the emergency room that evening and there was much debate whether to remove my spleen. They decided not to and instead kept me hospitalized for 10 days and then out of school for 3 more weeks after that waiting for it to heal on its own.