And we'll see why that's the case. It has to do with our immune system. Now, there are two different types of glycoprotein. So there are two different types of antigens. One of them we call antigen A or glycoprotein A, and the other one we call antigen B or glycoprotein B. And it's the presence or absence of these glycoproteins that determines and differentiates these blood groups from one another.