BLOOD GROUPS AND TRANSFUSIONS
Early attempts to transfer blood from one person to another produced varied results. Sometimes the person receiving the transfusion was aided by the procedure. At other times, the recipient suffered a blood reaction in which the red blood cells clumped together, obstructing vessels and producing other serious consequences.

Eventually, it was discovered that each individual has a particular combination of substances in his or her blood. Some of these substances react with those in another person's blood. These discoveries led to the development of procedures for typing blood. It is now known that safe transfusion of whole blood depend upon properly matching the blood type of the donors and recipients.

AGGLUTINOGENS AND AGGLUTININS
The clumping of red cells following a transfusion reaction is called agglutination. This phenomenon is due to the presence of glycoproteins called agglutinogens (antigens) in the red cell membranes and substances called agglutinins (antibodies) dissolved in the blood plasma.

Blood typing involves identifying the agglutinogens that are present in a person's red cells. Although many different agglutinogens are associated with human erythrocytes, only a few of them are likely to produce serious transfusion reactions. These include the agglutinogens of the ABO group and those of the RH group.

Avoiding the mixture of certain kinds of agglutinogens and agglutinins prevents adverse transfusions reactions. Such reactions are described in a subsequent section of this chapter.

ABO BLOOD GROUP
The ABO blood group is based upon the presence (or absence) of two major agglutinogens in the red cell membranes- agglutinogen A and agglutinogen B - which are present at birth as a result of inheritance. The erythrocytes of each person contain one of the four following combinations of agglutinogens: only A, only B, both A and B, or neither A nor B.

A person with only agglutinogen A is said to have type A blood; a person with only agglutinogen B has type B blood; one with both A and B has type AB blood; and one with neither agglutinogen A nor B has type O blood. Thus, all humans have one of four possible blood types - A, B, AB, or O.

Certain agglutinogens develop spontaneously in the plasma about two to eight months after birth. Specifically, whenever agglutinogen A is absent in the red blood cells, an agglutinin called anti-A develops; and whenever agglutinogen B is absent, an agglutinin called anti-B develops. Therefore, persons with type A blood have agglutinin anti-B in their plasma; those with type B blood have agglutinin anti-A; those with type AB blood have neither agglutinin; and those with type O blood have both agglutinin anti-A and anti-B.

Because an agglutinin of one kind will react with an agglutinogen of the same kind and cause red blood cells to clump together, such combinations must be avoided. The major concern in blood transfusion procedures is that the cells in the transfused blood not be agglutinated by the agglutinins in the recipients plasma. For this reason, a person with type A (anti-B) blood must not be given blood of type B or AB, because the red cells of both types would be agglutinated by the anti-B in the recipients type A blood. Likewise, a person with type B (anti-A) blood must not be given type A or AB blood, and person with type O (anti-A and anti-B) blood must not be given A, B, or AB blood.

Because type AB blood lacks both anti-A and anti-B agglutinins, it would appear that an AB person could receive a transfusion of blood of any other type. It should be noted, however, that type A (anti-B) blood, type B (anti-A) blood, and type O (anti-A and anti-B) blood still contain agglutinins (either anti-A or anti-B) that could cause agglutination of type AB cells. Similarly, because type O blood lacks agglutinogens A and B, it would seem that this type could be transfused into persons with blood of any other type. Type O blood, however, contains both anti-A and anti-B agglutinins, which can cause agglutination of types A, B or AB cells. Consequently, even for AB individuals, it is always best to use donor blood of the same type as the recipient blood.

Although the agglutinogens of the ABO and Rh groups are the ones most likely to cause serious blood transfusion reactions, every person's blood contains a number of other factors that sometimes cause problems. For this reason, it is good practice to determine whether samples of the recipient and donor blood will cause agglutination of the red blood cells before administering a transfusion to a patient. This procedure, called cross matching, involves mixing a suspension of donor cells in the recipient serum and then mixing a suspension of recipient cells in the donor serum. If the red blood cells do not agglutinate in either case, it is probably safe to give the transfusion.

RH BLOOD GROUP
The Rh blood group was named after the rhesus monkey, in which it was first studied. In humans this group includes several Rh agglutinogens (factors). The most important of these agglutinogen D; however if any Rh factors are present in the red cell membranes, the blood is said to be Rh positive. Conversely, if the red cells lack Rh agglutinogens, the blood is called Rh negative.

As in the case of agglutinogens A and B, the presence (or absence) of an Rh agglutinogen is an inherited trait. Unlike anti-A and anti-B, agglutinins for Rh (anti-Rh) do not appear spontaneously. Instead, they form only in Rh-negative persons in response to special stimulation's.

If a person with Rh-negative blood receives a transfusion of Rh-positive blood, the recipients anti-body-producing cells are stimulated by the presence of the Rh agglutinogen, and they begin producing an anti-Rh agglutinin. Generally no serious consequences result from this initial transfusion, but if the Rh-negative person - now sensitized to Rh-positive blood - receives another transfusion of Rh-positive blood some months later, the donors red cells are likely to agglutinate.

A related condition may occur when a woman with Rh-negative blood is pregnant with a Rh-positive fetus for the first time. Such a pregnancy may be uneventful; however, at the time of this infant's birth (or if a miscarriage occurs), the placental membrane, which separates the maternal blood from the fetal blood, may be broken, and some of the infants Rh-positive blood cells may get into the maternal circulation. These Rh-positive cells may then stimulate the maternal tissues to begin producing anti-Rh agglutinins.

If the mother, who has already developed anti-Rh agglutinin, becomes pregnant with a second Rh-positive fetus, these anti-Rh agglutinins can pass slowly through the placental membrane and react with the fetal red cells, causing them to agglutinate. The fetus then develops a condition called erythroblastosis fetalis (hemolytic disease of newborn).

AGGLUTINATION REACTIONS
In any blood reaction involving agglutinogens and agglutinins, the agglutinated red cells usually degenerate or are destroyed by phagocytic cells. At the same time, hemoglobin and other red cell contents are released, and the blood concentration of free hemoglobin increases greatly. Some of this hemoglobin may be phagocytized by macrophages and converted into bilirubin causing an increase of this bile pigment in the blood (hyperbilirubinemia). As a result, the tissues may develop a yellowish stain - a characteristic of the condition called jaundice (icterus).

Free hemoglobin also may pass into the kidneys and interfere with the vital functions of these organs, so that a person with a blood transfusion reaction may have kidney failure.

Infants with erythroblastosis fetalis are usually jaundiced and severely anemic. As their hematopoietic tissues respond to the need for more red cells, various immature nucleated erythrocytes, including erythroblasts, are released into the blood.

An affected infant may suffer permanent brain damage as a result of bilirubin precipitating in the brain tissues and injuring neurons. This condition is called kernicterus, and if the infant survives, it may have motor or sensory loses and exhibit mental deficiencies.