Scientists have put forth some theories to explain why mosquitoes swarm to some of us more than others, including one idea that differences in blood type must be to blame. Evidence is weak for this link, however, Vosshall says.
I’ve read that mosquitoes are attracted to “high sugar” (i.e. higher amount of polysaccharides) blood types, which O is. The Chinese even have a saying of people who are mosquito magnets: “You have sweet blood!” (I was told this a lot when I lived in China.)
That said, mosquitoes also seem more attracted to fresh meat, i.e. new and different smelling prey. Live in one locale for a while, and predation by mosquitoes seems to diminish. Perhaps it’s an ancient defense mechanism.
We have two kids to which the same dichotomy applies: I now need to check what blood type they have!