 So, the physiology of red blood cells, they're primarily made up of hemoglobin, as we talked about earlier. There are four heme groups in each red blood cell. You have normal hemoglobin, which is HbA, HbA2, and HbF. This HbF is fetal hemoglobin that you usually only see in fetus or shortly thereafter birth. We do also see some abnormal types of hemoglobin, specifically the sickle cell hemoglobin HbS, the hemoglobin C, and hemoglobin E disease, which give us HbC and HbE. HbC is a miscense mutation where lysine replaces glutamic acid at the sixth position. This is autosomal recessive, and carriers are usually unaffected or have a very mild disease. HbE is also a replacement of glutamic acid with lysine, just as in the HbC, however this is at the 26th position. Now let's look at the ABO blood groups that are seen with the red blood cells. You have four different blood groups, as well as a Rh factor. We'll talk about that in a little bit. With the blood groups, you have group A, group B, group AB, and group O. In group A, what you see is an A antigen that is present on the cell surface of the red blood cell. Group B shows a B antigen on the surface of the red blood cell. Group AB will show both antigens on the surface of the red blood cell, both A and B, and then you see in group O that there are no antigens here on the surface. So we have no antigens there. We will also see antibodies that are present in the plasma of all of these different types of red blood cell groups. With group A, you see anti-B antibodies, therefore those antibodies will attack any type of B group B blood. With group B, you'll see anti-A antibodies, meaning it will attack anything with an A antigen on the blood cell. Group AB cannot have any antibodies because if it would, it would attack itself and would no longer be able to survive, so it does not have any antibodies in the plasma. And group O has anti-A and anti-B antibodies, meaning that it cannot receive any A or B antigens in the blood. So what does this mean in practicality? Well we see that group A cannot receive any group B or AB blood due to the anti-B antibodies in the plasma. Anybody with a group B cannot receive any A blood or AB blood because of the anti-A antibodies in plasma. Blood type AB is considered a universal recipient. It does not have any of the antibodies in the plasma, therefore it has no worry of rejecting any type of blood. And then group O is a universal donor because it does not have any antigens on the surface of the cell. There is nothing in any body that will reject blood type O. Therefore it can be given to anyone. However, it is of note that group O cannot receive any blood type other than group O. They can donate to anyone else but they cannot receive anyone besides their own. Now let's look at the RH factor. Whenever you know someone's blood type, you will hear AB positive, AB negative, O positive, O negative. The negative and the positive are the RH factor. And what we're saying is that there is an absence or a presence of the RH factor on the surface of a red blood cell. So if there isn't a presence of the RH factor on the red blood cell, this will be a positive. If there is an absence, then it is a negative. So look over here at this red blood cell. You see in number one that there are RH factors on the surface of this red blood cell. So this will be a positive. And here there is nothing on the surface, so this is a negative. This is important in one particular group of people and that is pregnant women. We can have a disease called hemolytic disease of the newborn anytime that we see an RH negative mother that has an RH positive child. The mother would be exposed to fetal blood typically during birth and her body would then create anti-RH antibodies that can cross the placenta in subsequent births or subsequent pregnancies and it can attach itself to the blood of the fetus, which could then cause problems with that subsequent pregnancy. To treat that, we use anti-D immunoglobulin. This will be given to the mother in the third trimester as well as in the early postpartum.