What is the Rhesus (or Rh) factor?

If your blood is Rh positive, you have antigens in your blood which were first discovered in the Rhesus monkey (hence the name). If your blood is Rh negative, you do not have this type of antigen.

When someone needs a blood transfusion, it is very important to know their blood type. If Rh+ blood is given to someone with Rh- blood, their body will react to the unfamiliar antigens in the Rh+ blood. This would cause the two blood types to clump together. However, Rh- blood will not cause this problem for someone with Rh+ blood, because the Rh- blood does not have Rh antigens.

For the same reason, a person with type O blood should not receive type A blood. Type A blood has A antigens but not B antigens, type B has B antigens but not A antigens, type AB has both A and B antigens, and type O has neither.
If anyone receives blood containing antigens not already in their body, they are in serious trouble.
So let's hope no one makes a typeO about our blood types!

P.S. I forgot to mention that the Rh factor is not dependent on your A, B, AB, or O blood type, though it is similar