Mike Thorpe, age 13, of Rutland, Vt., for his question:
HOW MANY BLOOD TYPES DO WE HAVE?
Blood type in medicine is the classification of blood by the composition of its red blood cells. There are four types of blood: A, B, AB and 0.
Typing of red blood cells is a prerequisite for blood transfusion. In the early part of the 20th century, doctors discovered that blood transfusions often failed because the blood type of the recipient was not compatible with that of the donor.
In 1901 an American pathologist named Karl Landsteiner was the first to classify blood types. He also discovered that the blood types were transmitted by Mendelian heredity.
Blood type A contains red blood cells that have a substance A on their surface. This type of blood also contains an antibody directed against substance B, found on the red cells of persons with blood type B.
The blood type B contains the reverse combination. It contains any antibody directed against substance A.
Serum of blood type AB contains neither antibody, but red cells in this type of blood contain both A and B substance.
In type 0 blood, neither substance is present on the red cells, but the blood is capable of forming antibodies directed against red cells containing substances A or B.
If blood type A is transfused into a person with B type blood, anti A antibodies in the recipient will destroy the transfused A red cells. The opposite happens if blood type B is transfused into a peson with A type blood.
Because 0 type has neither substance on its red cells, it can be given successfully to almost any person. Persons with blood type AB have no antibodies and can receive any of the four types of blood. Because of this, blood types 0 and AB are called universal donors and universal recipients, respectively.
Other hereditary blood group systems have subsequently been discovered, but have much less significance for transfusion. Types M and N have importance in legal cases involving proof of paternity.
The heriditary blood constituent called Rh factor is of great importance in obstetrics and blood transfusions because it creates reactions that can threaten the life of newborn infants.
In 1940, Landsteiner and an American scientist named Alexander Wierner discovered the Rh factor. Most people have the Rh factor on the surface of their red blood cells. Their blood is called Rh positive. Persons who lack this antigen have Rh negative blood.
Harmful reactions may occur if an Rh negative patient receives more than one transfusion of Rh positive blood.
Doctors use blood tests to determine the number and proportion of the various kinds of blood cells. Other blood tests show the antibodies, chemicals, clotting factors and hormones in the blood.