People are divided into four main groups in respect of a certain reaction of the blood. This depends upon the capacity of the serum of one person’s blood to cause the red cells of another’s to stick together (agglutinate). The reaction depends on antigens (see ANTIGEN), known as agglutinogens, in the erythrocytes and on ANTIBODIES, known as agglutinins, in the serum. There are two of each, the agglutinogens being known as A and B. A person’s erythrocytes may have (1) no agglutinogens, (2) agglutinogen A, (3) agglutinogen B, (4) agglutinogens A and B: these are the four groups. Since the identiﬁcation of the ABO and Rhesus factors (see below), around 400 other antigens have been discovered, but they cause few problems over transfusions.
In blood transfusion, the person giving and the person receiving the blood must belong to the same blood group, or a dangerous reaction will take place from the agglutination that occurs when blood of a diﬀerent group is present. One exception is that group O Rhesus-negative blood can be used in an emergency for anybody.
Rhesus factor In addition to the A and B agglutinogens (or antigens), there is another one known as the Rhesus (or Rh) factor – so named because there is a similar antigen in the red blood corpuscles of the Rhesus monkey. About 84 per cent of the population have this Rh factor in their blood and are therefore known as ‘Rh-positive’. The remaining 16 per cent who do not possess the factor are known as ‘Rh-negative’.
The practical importance of the Rh factor is that, unlike the A and B agglutinogens, there are no naturally occurring Rh antibodies. However, such antibodies may develop in a Rh-negative person if the Rh antigen is introduced into his or her circulation. This can occur (a) if a Rh-negative person is given a transfusion of Rh-positive blood, and (b) if a Rh-negative mother married to a Rh-positive husband becomes pregnant and the fetus is Rh-positive. If the latter happens, the mother develops Rh antibodies which can pass into the fetal circulation, where they react with the baby’s Rh antigen and cause HAEMOLYTIC DISEASE of the fetus and newborn. This means that, untreated, the child may be stillborn or become jaundiced shortly after birth.
As about one in six expectant mothers is Rh-negative, a blood-group examination is now considered an essential part of the antenatal examination of a pregnant woman. All such Rh-negative expectant mothers are now given a ‘Rhesus card’ showing that they belong to the rhesus-negative blood group. This card should always be carried with them. Rh-positive blood should never be transfused to a Rh-negative girl or woman.