Originated from Wikipedia.org
Another characteristic of blood is Rhesus factor or Rh factor. It is named after the Rhesus Monkey, where the factor was first identified in 1940, again by Karl Landsteiner. Someone either has or does not have the Rh factor on the surface of their red blood cells. This is indicated as + or -, and the two groups are described as Rh positive (Rh+) or Rh negative (Rh-) respectively. This is often combined with the ABO type. Type O+ blood is most common, though in some areas type A prevails, and there are other areas in which as many as 80 percent of the people are type B.
Matching the Rhesus factor is very important, as mismatching (an Rh positive donor to an Rh negative recipient) may cause the production in the recipient of an antibody to the Rh(D) antigen, which could lead to subsequent hemolysis. This is of particular importance in females of or below childbearing age, where any subsequent pregnancy may be affected by the antibody produced. For one-off transfusions, particularly in older males, the use of Rh(D) positive blood in an Rh(D) negative individual (who has no atypical red cell antibodies) may be indicated if it is necessary to conserve Rh(D) negative stocks for more appropriate use. The converse is not true: Rh+ patients do not react to Rh- blood.
Rh disease occurs when an Rh negative mother who has already had an Rh positive child (or an accidental Rh+ blood transfusion) carries another Rh positive child. After the first pregnancy, the mother develops IgG antibodies against Rh+ red blood cells, which can cross the placenta and hemolyse the red cells of the second child. This reaction doesn't always occur and is less likely to occur if the child carries either the A or B antigen and the mother does not. In the past, Rh incompatibility could result in stillbirth or death of the mother. Rh incompatibility was until recently the most common cause of long term disability in the United States. At first, this was treated by transfusing the blood of infants who survived. At present, it can be treated with certain anti-Rh(+) antisera, the most common of which is Rhogam (anti-D). It can be anticipated by determining the blood type of every child of a RhD- mother; if it is Rh+, the mother is treated with anti-D to prevent development of antibodies against Rh+ red blood cells.
ABO blood type incompatibilities between the mother and child do not cause a similar problem because antibodies to the ABO blood groups are of the IgM type, which do not cross the placenta.