The RH factor is a blood type that is traced to the Rhesus Macaque.  This is why it's called the RH factor.  The reason why it's such a problem is because humans that live on the Earth (rather than under it, the inner earth humans have way different blood types), usually are RH- or RH+.  When an RH- father father's children with an RH+ woman, he father's children that carry his genes but are most likely RH+. 

However, when an RH+ father father's children with an RH- mother, the mother's natural immune system goes on the attack, seeking to reject the RH+ baby.  The only way around this is an infusion of blood into the baby -- or the woman to take drugs that could possibly destroy her liver.

This sort of thing happens when you try to cross donkeys with horses, where the female to be impregnated is a horse.  The horse's body seeks to reject the baby.  So this only happens when you cross species (such as horses and donkeys).  So, why does this happen to humans?

All humans living on the Earth look alike.  Races can cross with each other (A white man and a black woman can make children, and vice versa; a white man and an asian can make children and vice versa).  And there isn't any obvious way to tell the difference between one human and another except for extreme cases of skin disease (there is a case of a human having very scaly skin all over his body, it's a frightening sight).

The conclusion is inescapable.  There are two species of humans living on Earth.  One species has the Rh+ gene from the Rhesus macaque, and one species do not.  However, remember that there is over Thirty-three major blood group systems that are recognized by the International Society of Blood Transfusion.  However, the actions of what an Rh- mother can do to her Rh+ offspring is probably disturbing enough to consider that most likely there are two species of human on the face of the Earth.  And just because there is two species, does not mean we should practice racism.  And just because the Rh factor does exist, does not mean (although a lot of people think otherwise) that aliens came down to live and breed with us.

Where did the Rh factor actually come from?

There are other factors to consider:

Like, why does Color Blindness exist?

Why some babies are born blue?  Is it Cyanosis, Blue Baby Syndrome, or something else?

Why do some babies have tails?

Hypertrichosis, what does that mean?