When Europeans first experimented with blood transfusions in the 17th century, so many patients died that the procedure was outlawed in England, France, and Italy. It is said that the Incas in South America began transfusing blood much earlier, and that far fewer deaths resulted. If so, the reason, not understood at the time, may have been that nearly all of the Incas belonged to the same blood type, while the Europeans, like most groups of people, belonged to different – and incompatible – types.

Today, blood transfusion is safe only because blood samples from donor and recipient are tested to ensure that no dangerous transfusion reaction can occur from the mixing of incompatible blood.

Blood types, which are inherited, were first discovered in 1900 by an Austrian, Karl Landsteiner, who won a Nobel Prize for this life-saving medical advance. Since that time, other systems of classification have been devised, but Landsteiner’s is still the most important one.


What are the Main Blood Types?

In the ABO system, human blood is classified into four types: A, B, AB, and O. If your blood type is A, your red blood cells carry a protein called antigen A and your plasma, a protein called antibody b. (An antigen is a substance that stimulates the body to produce an antibody.) If you are type B, your blood contains antigen B and antibody a. Blood type AB carries both antigens but no antibody, while type O blood has neither of the antigens but both of the antibodies.

These categories are important in transfusion because certain antigens and antibodies are hostile to each other. Shaped so that they lock together, mutually hostile antigens and antibodies adhere in clumps that can cause fatal blood-vessel blockages.

Generally, people with type A blood can safely receive blood from A’s and O’s, while type B recipients are safe with blood from B’s and O’s. People whose blood is type AB are known as universal recipients, because their blood is compatible with types AB, A, B, and O. Type O people, on the other hand, are safe only with blood from type O donors, but they are themselves so-called universal donors, because they can give blood to anyone.

What is the Rh Factor?

The two most familiar antigens, A and B, are not the only ones to be found in human blood. Another antigen, which is known as the Rh factor, is contained in the red cells of more than 85 percent of the population of the world. This component becomes important when an Rh negative woman (a woman without Rh in her blood) conceives a child by an Rh positive man (a man whose blood contains Rh). Such cases pose a threat to the unborn child.

The danger comes from two possibilities: that the mother’s blood may contain antibodies hostile to the Rh factor and that these antibodies may cross the placenta and attack the Rh positive blood of the fetus. As a result, the child may develop jaundice, or may even be born dead.

How does the mother acquire those destructive antibodies? She may get them through blood transfusion. It is also possible that her body will develop them if the blood of her fetus seeps into her own bloodstream during childbirth or miscarriage.

The first pregnancy poses little risk, because it takes time for the antibodies to build up. But once the mother’s blood has become sensitized, Rh incompatibility may be a problem in the next pregnancy.

To forestall the danger, doctors can administer a recently developed anti-Rh antibody to Rh negative mothers after each of their pregnancies so that their blood will not create antibodies hostile to Rh positive blood. Another modern technique is the exchange blood transfusion, in which all of a fetus’s blood is replaced.

What Gets Counted in a Blood Count?

In a routine count the blood’s basic components – red cells, and platelets – are enumerated from a small sample of whole blood. Each number or percentage is then compared against a standard to check for deficiency or excess.

A more detailed blood count breaks down the relative numbers of the six specialized kinds of white cells. Abnormalities may indicate allergic reaction, infection, or blood disease.

A blood test begins with a fresh blood sample, which is diluted with distilled water and salts so that the concentrated cells separate. A known volume of the diluted sample is spread evenly over a grid under a glass cover, and the number of each kind of cell is counted under a microscope. In this way, a hematologist arrives at a number – the person’s count for a particular type of cell. Today, electronic scanners can perform this task.

Can You Give Yourself a Blood Transfusion?

Blood transfusions have saved perhaps a million and a half American lives – more than the number lost in all the wars the United States has fought. Transfusions, however, are not without their dangers. They can transmit a disease from the person who gives the blood to the one who receives it, and if the blood of donor and recipient are not matched properly to be sure they are compatible, transfusions can cause troublesome and even fatal reactions.

The best way to guard against these potential hazards is for a patient to give blood to himself. Of course, this protective measure is impossible in emergency-surgery situations; it can be used only when a patient knows weeks or months in advance that he faces an operation. In that case, the patient goes to the hospital at intervals before the operation and allows technicians to take 1 pint (.47 liter) of blood at a time until several pints have been drawn. The hospital then stores the blood and sends it to the operating room on the day of surgery.

Can Blood Type Prove Paternity?

Heredity determines the blood type of every individual. A child inherits two blood-type genes, one from each parent, and cannot belong to any blood type unless at least one of his parents carries a gene for that type. Dissimilar blood type in two people may thus constitute evidence that they cannot be parent and child. Accordingly, blood types are often cited in paternity suits or in the case of a possible switch of two newborns in the hospital.

But even when a child and an alleged father have the same blood type, this proves only that the adult could be the child’s father, not that he is the father: theoretically, countless other males with the same blood type could have fathered the child.

To take a simple example: Suppose a child has type A blood and its mother type O. If the supposed father also belongs to type O, he cannot possibly be the father, because he has no gene for blood type A to pas on to his offspring. But if the alleged father belongs to type A, then the child could, indeed, be his.