Early experiments with human blood transfusion often resulted in the death of the patient for unknown reasons. In 1901, it was discovered that there were three* blood types, A, B, and O, and that mixing blood from different types caused an immune response that resulted in clumping.

*Type AB is rare and was discovered later.

ABO Blood Type: An individual's red blood cells will contain proteins of type A, or B, or both, or neither. The body produces antibodies that will attack any foreign type. Alleles of types I_A and I_B are dominant over type i.

Rh Factor: The Rh factor, the second most important blood group system after the ABO blood group system, was first discovered in rhesis monkeys. The Rh factor is inherited independently from the ABO blood type. Genotypes for the Rh factor are +/+, +/-, and -/-. People who are +/+ or +/- possess the Rh(D) antigen and test as Rh positive. People who are -/- do not posess the Rh(D) antigen and test as Rh negative. About 15% of Americans are Rh negative.

Rh Sensitization: One interesting medical scenario involves an Rh negative mother who carries an Rh positive baby. (The baby of an Rh positive father and an Rh negative mother can be +/- or -/-.) If the baby is +/-, the first pregnancy causes Rh sensitization in the mother, because she is exposed to foreign proteins and builds up antibodies against them. Future pregnancies can be increasingly difficult, as the mother's antibodies attack the baby.

Donors and Recipients: Because the red blood cells of blood Type O negative people contain no proteins that could be rejected, Type O negative is referred to as the universal donor. Because red blood cells of Type AB positive have all possible antigens (proteins), none will be seen as foreign so Type AB positive is the universal recipient. Since the plasma (the liquid part of the blood, from which the red and white cells have been removed by centrifuging) of Type AB positive people contains no antibodies, the plasma of Type AB positive donors is universal. In an emergency, where there isn't time to do a blood type test, knowing who is a universal donor could save lives.

Racial Differences: Differences in the frequency of blood types in different populations of humans are presumably due to historical isolation of small populations, which led to genetic drift. Lacking any scientific merit, racial segregation of blood remained the policy of the American Red Cross until 1950, despite the protestations of a prominent African American physician and transfusion researcher, Dr. Charles Drew. For some good reading on misconceptions about race and human differences, read R.C. Lewontin or Steven Jay Gould.

Blood Type Genetics Problems