One theory that I hear, which strikes me as somewhat true is that humans with O- blood are a slightly different species than other humans. I’m speaking metaphorically on that, to try and make my point understandable.
Because the Rh (-) is not naturally found in the human race and there is no confirmation as to how it became part of the human race. So, those who are Rh (-) are a slightly different species of human than other humans.
It may not be so much as a superior immune system at work as simply that they are slightly less compatible with virus that can normally affect the human race.
BOL! My mother, who was 0- and never had any flu and seldom had colds was a teacher for most of her adult life. She and my O-negative sibling seldom had allergies or colds.
She was a teacher back in the 1940-s & for 60+ years. With the exception of an emergency gyn surgery when she was 40 something, she never had allergies nor sinus problems like my Dad and I did. She did have adult onset arthritis in her hands after turning 80.