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In: Biology
The main blood groups are the ABO and Rh systems (+ or -), so you are either A, AB, B, O with a plus or minus after. When we say someone is group A, B, AB, or O, we are talking about the proteins that are expressed on the surface of the red blood cells. Actually, A, B, AB and O all express the same protein, but A and B are cause by an enzyme that is inherited from your parents that will modify that universal protein a little bit differently, making it different enough that if you don’t make it, your body will recognize it as foreign. Why this is clinically important is we naturally form antibodies against the blood groups we don’t express – even if you’ve never been exposed to foreign blood, you have these antibodies. This is thought to be due to the interaction of some of our gut microbes with our immune system, but if you are group O, you will express antibodies to A blood and B blood. If you are group A, you will form antibodies against B blood (but not O, O is just the absence of A or B), and if you are group B, you will form antibodies against A blood. What this means is if you get an “incompatible” blood product (something you have antibodies against), those pre-formed antibodies will bind to the foreign red blood cells (RBCs) and target them for destruction. This process is called hemolysis, and all the contents inside those RBCs get released and cause major problems, usually death. This is so important that if you need blood, they will test your blood both to see what kind of blood you make, and what kind of antibodies you make. You should be able to infer the blood type from both methods, and if there is any discrepancy it needs to be investigated. Until this information is obtained, we will give “universal” blood, group O, since there is no such thing as anti-O antibodies, anyone can receive it (which is also why if you are type O, your blood is extremely important and you should consider becoming a blood donor). Once the blood type of the patient is known, they will switch to “compatible” blood, which will either be your same type, or something close to it that won’t cause any issues (ie if you are group AB, they will give A, B, or AB depending on what they have available).
The Rh (+ or -) system is a little different. You don’t naturally make antibodies to it, you need to be exposed. So in an emergency, they may give “incompatible” units, which wouldn’t cause issues in the short term, but you would now be sensitized and have antibodies against that blood group, meaning you couldn’t receive it again. This is most important for women of childbearing age. The antibodies against Rh are a different kind than the A and B antibodies, and they can cross the placenta. So if a Rh- mom is carrying an Rh+ baby, if she has been “sensitized” in the past (either from receiving an “out of group” blood transfusion or during a prior pregnancy), there can be serious complications, including death of the baby. A lot of still births used to be due to this, but now if a pregnant woman is Rh-, we automatically give a drug to prevent her from making those antibodies.
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