I'm not going to get the medical terms exactly right here, but in it's simplest form blood is O negative is the "base", then other antigens and factors are included. People with type A or B have one antigen and AB has 2 antigens. Which is why O- is the universal donor (everyone can recieve O- without the body seeing it as foriegn) and why AB+ is the universal recieptent but O- must recieve O-. Positive people can recieve negative blood types with fewer or equal antigens since it includes nothing to reject, but the "postive" factor causes the body of a person with negative blood to see it as foriegn.
Pregnant women who are RH negative are given Winrho in about the 20th week in case the baby is postive to prevent complications. If the baby is born postive, then Mom is given another shot in case they chose to have another child.
Truthfully the docs are not completely sure of all the complex interactions that causes WinRho to increase platlets, but they know it seems to bind with red blood cells and flush them from the body for individual who still have their spleen and their blood typ is positive. However, it is why they also monitor for the risk of anemia as the previous poster noted.
"I am an old man and have known a great many troubles, but most of them have never happened.\" — Mark Twain\\\\\\"Worry is a misuse of the imagination.\" — Dan Zadra