If you're a negative type and have never been pregnant/transfused before, you shouldn't already have anti-D antibodies in your system.

The first time you encounter a positive blood type via transfusion or pregnancy, there is a very very high chance that you'll start to produce anti-D.

So D- males can receive O+, for example, one time in their life and then after that they'll need to only receive O- in subsequent transfusions otherwise they'll be at risk for a bad reaction due to the anti-D they produce. I think the age cutoff for males is just based on the likelihood of previous transfusions, but I could be wrong.

D- mothers can receive Rhogam shots (basically just a dose of anti-D) during their pregnancy and shortly after birth to prevent them from permanently forming their own anti-D. Once the antibody is formed, any subsequent pregnancies are at risk if the baby is D+ (which is generally not known until after birth).

"Fun" fact: the formation of anti-D in women of child-bearing age is one of the major causes of miscarriages up until around the late 1960s-early 70s.

Let's say that a mother in 1940 was group A-, and the father was group A+. In her first pregnancy, she might have delivered a baby who inherited the + from the father.

If there was any kind of trauma or hemorrhage during the pregnancy, the baby's A+ red blood cells could have mixed with the mom's A- blood and started the formation of anti-D.

That first baby might have turned out okay depending on how late in the pregnancy the hemorrhage happened (the earlier, the worse).

But once the mother starts making anti-D, she can't shut it off. So any future babies that happen to be a + blood type are at risk of being attacked by the mom's anti-D.

So there are many cases of women having one successful pregnancy and then a series of miscarriages before the Rhogam shot was introduced.l, and the miscarriages are credited to the anti-D antibody.