Human red blood cells carry markers on their cell surface. These markers are antigens. We call these markers antigens because they will elicit an immune response and produce antibodies. A "B" red blood cell (a red blood cell carrying the B antigen) will be destroyed by the B antibody. Similarly, a red blood cell carrying the A antigen will be destroyed by the A antibody. RBC is short for Red Blood Cell.  For my blog post reviewing antigens and antibodies, click here.

People that are blood type B carry antibodies to A. This is why we can not give Type A blood to a Type B person. The A antibodies present in the patients own blood will attack the donated Type A blood. This is the same for people with Type A blood. These people make antibodies that will attack Type B blood. Therefore we can not give Type B blood to a Type A person.

If only we could stop there, but we can't. There are two more variations of human blood type when considering the ABO blood system: Type AB and Type O. People with Type AB blood have A and B antigens on their own blood cells, so they can't have antibodies to either. If they did, those antibodies would constantly attack the person's own red blood cells. So Type AB people have no antibodies present in their blood and can receive blood of any blood type. These people are called universal recipients.

Type O blood does not have the A or B antigen present on the red blood cells. Because of this reason, Type O blood can be given to any person. People with Type O blood are therefore called universal donors. Because there are no antigens present on the donated red blood cells, patients that receive this blood will not have a reaction to the donated red biood cells.

Now wait, you are probably saying to yourself, Type O blood carries antibodies to both A and B red blood cells. Won't the antibodies present in the donated Type O blood cause a reaction to the patient's own blood (if they are Type A, Type B or Type AB)?   The answer to that is yes, there will indeed be a reaction. However because of shear quantity (very few antibodies A and B when compared to the entire volume of the patients blood), this reaction is not significant.

Here is a chart to help you remember the important parts from today's lesson: