It’s not even white blood cell’s job to kill cancer cells, it’s killer T cells’ job, so why is White Blood Cell apologising?

I think White Blood Cell was originally conceived to represent the cellular immune system in general, whose job is basically to protect the body. Plus, the author was trying to establish a touching moment with Cancer Cell, and could you really see Killer T having that conversation with his established personality?

It's more for narrative purposes than it is for accuracy, I think.