Mary Marsh, age 13, of Albany, New York, for her question:
What exactly are blood types?
Ages ago, people sensed that there is more to their blood than meets the eye. Old expressions such as blood relatives, blood brothers and life's blood suggest bonds of kinship and friendship and even a symbol for life itself. Scientists do not pooh pooh these mystical old notions because the discovery of blood types proves that there is at least some truth in them.
When a wounded warrior lay bleeding, friends and relatives yearned to give him their own life's blood. Later, doctors injected the blood of donors into the veins of their bleeding patients. Sometimes it saved a life. Sometimes the donor's blood caused the patient's blood to clot and proved fatal. Around 1900, a team of researchers discovered why some blood types are compatible and others incompatible.
The secret is in the multitude of complex biochemicals that circulate in every body's blood stream. Some are carried in the floating red cells, others in the liquid plasma. In this case, two opposite classes of biochemicals are involved the antigens and the antibodies. The various antibodies attack certain antigens as hostile invaders.
The antibodies provide the body's immunity system. However, when a patient gets the wrong transfusion of whole blood, his antibodies attack the donor's antigens. This causes his red cells to cling and clump together and clog his blood vessels.
Researchers let various blood samples stand until the red cells sank to the bottom of the liquid plasma. When they mixed samples of red cells and plasma, some pairs clotted and some did not. The results were sorted into four blood types. The clotting factors were identified as A and B, two antigens that may or may not be present on the surfaces of the red cells. The four basic types are A and B, AB which has both antigens and 0, which has neither.
Every blood type has a multitude of defending antibodies, but none carries one that attacks its own antigens 0 type, with neither A nor B, is the universal donor, acceptable to all types.
Though all human blood is very much alike, the ABO system and other slight differences are very important. The rare Rh factor in a mother may be incompatible with the blood of her unborn baby. And only persons with the strange "t" factor may be able to taste the bitterness in a certain chemical.
Researchers soon learned that the A and B factors are carried by genes that must obey the laws of heredity. So there is some truth in the old idea of family, or blood relationships. Most human beings are 0 type. The rarer types are more common in some national groups and almost absent in others. Some researchers suspect that people of certain blood types tend to be more prone to certain diseases, though these investigations are hard to verify and still incomplete.