You’ve probably heard that each person’s blood can be classified according to blood type. The term refers to a relatively minor characteristic of blood—specifically, the types of proteins that stud the surface of your red blood cells. This detail is important because
it influences how your immune system differentiates friendly blood cells from potential enemies. If your immune system discovers red blood cells that don’t match its expectations, it’s likely to destroy them.
Usually, there’s little reason to spend time thinking about your blood type. But it becomes critically important in one situation—if you lose a lot of blood and need to be topped up with someone else’s. In this situation, the donor’s blood type needs to be compatible with yours, or your body will launch a dangerous, debilitating war against the new blood cells.
More rarely, blood-type issues can occur during pregnancy. In some cases, an unborn child inherits a blood type from the father that’s incompatible with the mother’s. This in itself doesn’t pose a problem, but when the mother is exposed to this blood (typically during delivery), her immune system may begin building antibodies that can destroy the foreign blood cells. Because these antibodies linger in her blood forever—they’re essentially a stockpile of on-reserve weapons—they can cause problems for future babies in future pregnancies. Fortunately, modern medicine deals painlessly with this issue: Early in pregnancy, mothers are given a blood test and, if necessary, a vaccine that prevents the production of these antibodies.
Source of Information : Oreilly - Your Body Missing Manual