Why is O a universal donor

From an early age, we can usually name blood groups and we know that the AB group is a universal recipient and O is a universal donor. We later find out that there is a Rh system that also plays an important role not only in blood transfusions. In fact, we know 39 blood systems to this day. Why hasn't anyone told us about the others and is O really universal donor?

To understand the blood systems, we must immerse ourselves in the circulatory and immune systems. Blood consists of, among other things, three main solid components, i.e. red blood cells, white blood cells, and platelets. Red blood cells are used to transmit breathing gases, white blood cells are used to fight foreign substances (antigens), and platelets help to stop bleeding. In particular, red blood cells but also platelets, have a number of proteins on their surface that can act as antigens, i.e. substances that elicit an immune response. The problem is that there are hundreds of these antigens and different people have them in different combinations.

The definition of the blood system is relatively complex and requires at least a basic knowledge of genetics. Briefly, however, one or more antigens form the blood system if these antigens are encoded by genes at the same locus, i.e. at the same part of DNA, or are encoded by very similar homologous genes, i.e. genes that share a common ancestor.^[1]

The more frequently mentioned blood systems include ABO, Rh, MNS, Kell or, for example, Duffy. The ABO system contains antigens A and B and according to the presence of these antigens on the surface of the red blood cell we recognize blood groups A, B, AB and O, where O indicates the absence of both antigens. The absence of any of the antigens also indicates the presence of an antibody to that antigen in the blood plasma. The encounter of an antigen with an antibody leads to the induction of an immune response.

The AB0 and Rh systems are very important because their antigens can cause a so-called acute hemolytic reaction in the recipient if the blood is incompatible. Consistency in other systems is less important because either it does not lead to a severe immune response or the proportion of the antigens in the population is extremely low or extremely high. Problems can occur only in about 3 to 6% of cases. Consistency in other blood systems is more important in patients with anemia, who require more frequent transfusions, in women under 45 years of age due to the risks of pregnancy and blood transfusions, where donors and recipients come from different parts of the world.^[2]

However, blood group O is relatively rare. Europoid people have up to 8% of blood group O, but in the Asian population, for example, it is only 0.3%. Generally speaking, O is not a universal donor, but if an individual is additionally RhD negative and if blood is donated to an ethnically related person, it is considered universal.

Finally, it is worth noting that in the future, transfusion centers may work without blood donors. Already in 2008, researchers developed an artificial red blood cell^[3], and in 2017 they succeeded in growing a blood stem cell that can become a red blood cell, a white blood cell, or a platelet^[4]. Artificial blood would also have another huge advantage in preventing the transmission of pathogens, such as HIV, HBV, and HBC, which enter the recipient's body due to insufficient control of the donor's blood.