Definition: what is a blood group? When an ill person needs blood, they can’t just receive any type of blood, but they have to receive blood that corresponds to their blood group. Although all blood has the same cellular structure, there is a variability or “polymorphism” of some blood elements among individuals which makes blood transfusions impossible within certain groups of people. Blood groups are a classification of blood which relies on the presence or absence of certain elements called antigens which lie on the surface of red blood cells. There are four blood groups determined on the basis of an AOB classification of the subject’s blood: A, B, AB and O. But there are actually two types of antigen: A and B. The distinguishable groups are: - for group A, the individual has A antigens on the surface of the red blood cells (as well as anti-B antibodies in the blood stream) - for group B, the individual has B antigens on the surface of the red blood cells (as well as anti-A antibodies in the blood) - for group AB, the individual has both A and B antigens on the surface of the red blood cells (but no antigens) - finally, for group O, the individual doesn’t have any antigens (but possesses antibodies for both anti-A and anti-B in the blood stream) Compatibility between blood groups In the majority of cases, patients receive red blood cells from a donor of the same blood group. This is because, receiving blood of a different blood group can have serious consequences for the patient. This is due to the presence of antibodies mentioned above. These elements attack the antigens which aren’t present in the blood of the individual. For example, if you transfer blood from group B to an individual of group A, the anti-B antibodies can cause an immune response which can lead to the destruction of the red blood cells that have just been transfused. Consequences vary from mild (shivering, anxiety) to serious (kidney failures) and could lead to death. Group A can therefore only be given to people who are also group A and group B only to group B. Group AB can however be given to group A or group B. Group O can be given to any group, so individuals with group O- are considered “universal donors”. The individuals with AB+ have been considered “universal recipients”. The rhesus factor: what is it? However, the blood group isn’t the only thing that’s important, there is also a category: rhesus. Rhesus refers to a red blood cell antigen that is found in their wall. It makes it possible to determine two different blood group systems: Rh positive (Rh+) and Rh negative (Rh-). The people who are Rh+ are those who have this antigen, and this is the case in the majority of the population. Rh- refers to people who are lacking the antigen. The rhesus factor is also important to be able to determine if a blood transfusion is possible between two people. Blood transfusions can be “iso-rhesus” or type specific, meaning between Rh+ and Rh- but only in one sense: Rh- can be given to Rh+, but the Rh+ can’t be given to Rh-. Again, this is due to of a presence of antibodies aimed at the antigen in people who are Rh-. In Rh- pregnant women whose foetus is Rh+, this incompatibility can lead to complications at birth, such as a haemolytic disease in the new-born during childbirth. Frequency of blood groups Blood groups are dictated by the presence of an allele in a particular gene of the chromosome 9 meaning a child’s blood group usually depends on the group of their parents. Two parents of group A will mean the child is also group A, and similarly with those of group B. If one of the parents is A and the other is B, the child will be AB. Finally, if one of the parents is O and the other is A or B, the child will be A or B, with O not being a dominant group. Rhesus works in the same way with Rh+ being dominant over Rh-. In France, the most common blood group is A+, closely followed by O+ and the rarest is AB-. But these statistics vary from one population to another.