are there any benefits with regard to immune system or any other aspect of physiology for people who have a certain blood type over another (other than universal donor/reciever)?
does anyone know of studies, or are there too many variables for a quesiton like this?
Other than the universal donor/receiver, no, there aren’t.
If this is a question about the supposed blood type diets, then most emphatically no.
Well, people with blood group O have a lower risk of developing cancer of the stomach, colon, uterus, cervix, and gall bladder (reference here - last three paragraphs).
Following up on the above, are particular blood groups more or less likely to be associated with particular ethnic groups, who in turn might be more or less prone to certain diseases either by genetics or as a result of diet and other cultural factors?
If the disease is linked to diet or other cultural factors, then any correlation with blood type is purely coincidental. You would have to show that people of other ethnicities, with the same blood type but different diets or lifestyles, had the same risk for the disease. Very unlikely.