My father grew up on a farm in a part of the world with a lot of mosquitos. He says he would work in the fields all summer shirtless and never even notice any mosquitos.

I hate going there in the summer because of the little bastards. (And the horseflies, I really hate them).

Wind forward 50 years of mostly city living in other parts of the world, and whenever he flies back to his childhood home for a few weeks holiday he gets eaten alive.

His blood type has not changed. He may be more sweaty as he is a little chubbier, but he was probably quite sweaty working in the fields as a kid as well.

I suspect it has to do with lifestyle. For him (and me) mostly working in office environments for most of the year for many years, makes your skin/odour/heat different than if you were an outdoorsy person that has a more hardened skin that is less attractive to mossies. Perhaps.

That said I don't really hear of anyone that lives in the mosquito heavy area that is susceptible to them whether they are office or farm people. Only outsiders. That may be self-selecting as they would move away though.