Blood Types

Blood types or groups are differences between people's blood that can be recognized by the immune system. Complex chemical substances found on the surface of red cells are different for each blood group. When donor red blood cells carrying a certain blood group are transfused into a recipient who lacks that group, the recipient may make an antibody that binds to or 'labels' that blood group or 'blood group antigen' as foreign. This may cause the donor red blood cells to be destroyed in a 'transfusion reaction'. Related blood groups are organized into 'blood group systems'. 

There are over 600 different blood groups, but only a few of them are important for routine transfusions. The two most important ways of classifying blood in transfusion work are a person's ABO and Rh groups.

Within the ABO system, people can be one of four groups: O, A, B, or AB; with respect to Rh a person is either positive or negative. This creates a total of eight main blood types.

ABO Types

The ABO blood types were discovered by Landsteiner around 1900. He received the Nobel prize for the discovery, which eventually changed transfusion from a desperate measure to a routine medical intervention.

A person's ABO type is determined by the presence or absence of the A or B substance on his red cells. A person can have one or the other (types A or B respectively), both (type AB), or neither (type O).

The Rh Factor

A blood protein which was discovered in 1940. Those who have this "Rhesus" factor on their red blood cells are considered Rh positive. Those who lack  the factor are called Rh negative. Thus every person is either Rh positive or Rh negative while also belonging to one of the ABO groups. Approximately 85% of the population is Rh positive.

Transfusion Reactions

Blood typing prevents transfusion reactions which are caused by blood group antibodies. Thus, someone with blood type A does not have the B group on his red cells, and always makes an antibody against B. If he receives blood from someone with B or AB blood his anti-B will destroy the transfused red cells, a serious reaction that, in the worst case, can lead to death.

Rh negative persons don't automatically make anti-Rh, but if they receive Rh-positive blood, most will then make the antibody that will destroy Rh-positive red cells if they are transfused a second time. Problems with the Rh factor also occur during pregnancy. If a baby is positive and the mother is negative, she may make anti-Rh in response to the baby’s blood. This  occurs because a few of the baby's red cells cross the placenta into the mother's circulation, particularly during labor. Because of this timing, the first pregnancy is not affected: it is in subsequent pregnancies that the antibodies in the mother’s body can destroy the baby’s red blood cells.