Why Are There Different Blood Types?

Different blood groups are the result of genetic variations that determine the presence or absence of specific antigens on the surface of red blood cells. These variations arise due to differences in genes that are inherited from parents, and they have evolved over time to serve various biological and immunological functions, such as influencing immune responses to infection and, in some instances, providing resistance to disease.

The ABO blood group system, discovered by Karl Landsteiner in 1901, is one of the most well-known systems. It classifies blood into four types—A, B, AB, and O—based on the presence or absence of A and B antigens. For people who have type A blood, their red cells express A antigens, and they produce antibodies against B antigens. Knowledge of ABO blood type is essential for blood transfusions. If type A blood is mixed with type B blood, for example, anti-B antibodies attack the type B red blood cells, causing a transfusion reaction. 

The Rh blood group system, discovered by Landsteiner in 1940, adds another layer of complexity. It is classified according to the presence or absence of the Rh antigen, often referred to as the Rh factor. The presence of the Rh factor poses a risk for individuals who are Rh-negative, if they receive a transfusion with Rh-positive blood. Likewise, Rh incompatibility can pose a serious threat in pregnancy, potentially harming the fetus or newborn.

The distribution of blood types varies globally, influenced by genetic drift and natural selection. For instance, when the red cells of a person with the sickle-cell trait, the cause of sickle-cell anemia, are invaded by the parasite that causes malaria, the infected red cells and the parasite within them are destroyed. In this way, the individual is resistant to malaria. The geographic distribution of this trait, occurring primarily in a broad equatorial belt in Africa, is explained by the fact that heterozygous persons are resistant to malaria.