What exactly they lose depends on the species, human red blood cells indeed discard organelles and the nucleus.
But having organelles and a nucleus are not the formal definition of a cell. To begin with, any prokaryotes won’t have a nucleus, and organelles aren’t mandatory.
They don’t have the organelles or nucleus that most cells do have. They can’t replicate.
I mean like most fields it depends on your definition. It doesn’t help that many people are taught they aren’t “true” cells.
What exactly they lose depends on the species, human red blood cells indeed discard organelles and the nucleus.
But having organelles and a nucleus are not the formal definition of a cell. To begin with, any prokaryotes won’t have a nucleus, and organelles aren’t mandatory.