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Blood types are relevant to pregnancy and probably also to COVID-19. To delve into this topic, let’s begin by discussing what we mean when we talk about blood types. Your blood contains trillions of cells, most of which are red blood cells (RBCs, also called erythrocytes). These cells are packed to the hilt with the protein hemoglobin, which carries nearly all of the oxygen that travels in the blood to body cells and also carries a portion of the carbon dioxide that body cells release. However, surrounding each RBC is a cell membrane, and that membrane has different molecules on its surface. Some of these surface molecules are made of sugars and are antigenic, meaning that that are capable of stimulating the immune system. When your immune system is developing, it learns that the various antigens (things that can stimulate an immune response) on all of your cells are part of you and that it consequently should not attack those antigens. However, your immune system does not learn to avoid attacking antigens that are not part of you. In the case of blood, the problem comes in when the immune system of one person is exposed to blood cells with antigens that are foreign, because those cells come from somebody else. The somebody else can be a blood donor, or it can be a fetus whose blood accidentally mixes slightly with the blood of the mother. If this happens once, there is not enough time for the mother’s immune system to mount much of a response, but the encounter primes the mother’s immune system. Then, if she gets pregnant later with another baby with the same antigens on its blood cells as the older baby, the new baby’s cells will be attacked, if there is some mixing of fetal and maternal blood.
With ABO maternal fetal incompatibility, the consequences for the baby generally are not as severe as with Rh incompatibility, but such newborns can develop neonatal jaundice (yellow skin) and anemia, resulting from maternal antibodies crossing the placenta and destroying fetal RBCs.
This phenomenon is called maternal fetal blood incompatibility. It can happen in cases when the mother is negative for a blood cell antigen called Rh and her babies are positive for it. This is called Rh incompatibility, it’s preventable with a therapeutic antibody called RhoGAM, but if it’s not prevented, it can cause very severe problems for the newborn, including brain damage.
Incompatibility also can happen in connection with the ABO blood types. The ABO typing system is particularly important in connection with blood donation and is the result of the two different antigens, one called A and the other called B. People with blood type A have the A antigen on the surface of their RBCs. People with type B blood have B antigen on their RBCs. It’s possible to be type AB, meaning that your RBCs have both A and B antigens. However, the most common blood type is type O in which RBCs have neither the A nor the B antigen. In such people, the immune system treats the A and B antigens as foreign and thus mounts a response if they receive blood from donors of type A, type B, or type AB, so they can receive blood cells only from donors who are also type O. Now, similar to Rh incompatibility, sometimes when a mother has type O blood, there can be a reaction if her baby has one of the other types, most commonly if it’s type A. With ABO maternal fetal incompatibility, the consequences for the baby generally are not as severe as with Rh incompatibility, but such newborns can develop neonatal jaundice (yellow skin) and anemia, resulting from maternal antibodies crossing the placenta and destroying fetal RBCs.
Also related to the ABO typing is some research suggesting that people with type O blood enjoy a reduced risk of developing severe complications from COVID-19 compared with people of other blood types. Currently, scientists are studying this issue and the possibility that it is related to problems with the body’s clotting system that also seem to be involved in the COVID-19 disease process. One possible reason, or part of the reason, for the observed lower risk of COVID-19 complications with blood type O may be that people with this blood type are faster at breaking down a protein called von Willebrand factor. This is a protein that helps the body form clots that the body makes during pregnancy a little bit more than it does when you are not pregnant.
