I am A- bloodtype, and yesterday I was thinking about that and decided it must've, in the past, been rather helpful. When my mum was born she was blue, and her grandmother said, "This baby's dead!" She revived, though, and from that story I decided Mummy Barbara must've been Rh- too. I think in the past being able to avoid having one child after another, with all the attendant risks of childbed, would've led to a longer life. Generally the woman has one child (if her mate is Rh+) then her body is sensitised to the alien Rh factor and attacks subsequent fetuses. It seems to be similar to what they say about people heterozygous for the sickle cell anaemia alleles (if that's how one should phrase it) having a resistance to malaria.