Red blood cells (called erythrocytes) have a type of antigen on their surface. Composed of sugar molecules, these antigens are called agglutinogens. There are two types of agglutinogens: type A and type B. The type of antigen on the surface of your red blood cells determines your blood type.
There are four basic blood types, made up from combinations of the type A and type B antigens.
Type A: The red blood cells have the type A agglutinogen.
Type B: The red blood cells have the type B agglutinogen.
Type AB: The red blood cells have both type A and type B agglutinogens.
Type O: The red blood cells do not have any agglutinogens at all.
There is another protein (called Rh factor) that is sometimes found on red blood cells. If a person has Rh factor, their blood type is called “Rh positive.” An individual lacking this protein is called “Rh negative.” Combined with the ABO blood types described above, a person may be A+, A-, B+, B-, AB+, AB-, O+, or O-.
Blood types vary depending on the geographical region: Scandinavians have a high probability of carrying the A blood type, while those indigenous to central Asia are more likely to carry the B blood type. The O blood type is the most common blood type around the world.
According to the National Center for Biotechnology Information (a molecular biology resource funded by the government), the breakdown of blood type by region is:
Blood Type A: Central and Eastern Europe
The A blood group is common in central Europe. Nearly half the population in Denmark, Norway, Austria, and the Ukraine have this blood type. This blood type is also found in high levels among small, unrelated groups of people. In Montana, 80% of the Blackfoot tribe has the A blood group.
Blood Type B: Asia
The B blood type is rare in Europe (about 10% of the population), but fairly common in Asia. Nearly 25% of the Chinese population demonstrates this blood type. This blood type is also fairly common in India and other Central Asian countries.
Blood Type AB: Asia
The AB blood type is the rarest of all. It is found in up to 10% of the population in Japan, Korea, and China, but is extremely rare in other regions.
Blood Type O: The Americas
The O blood type is the most common around the globe, and is carried by nearly 100% of those living in South America. It is the most common blood type among Australian Aborigines, Celts, those living in Western Europe, and in the United States.
The majority of people in any geographical region are Rh positive. Caucasians are the most likely to be Rh negative, with approximately 17% of blood donors demonstrating a lack of this protein. Native Americans are the next highest proportion of the population to test as Rh negative: approximately 10% of donors from this population lack this protein.
As World War II raged through the Pacific, blood products were in short supply. In emergency situations, Japanese and British medics would resort to coconut water. Coconut water (the juice inside a young coconut, not “milk” which is made from grinding up the meat of the fruit) has fewer electrolytes than blood plasma, but it is sterile and works in a similar manner to a saline IV drip. In a pinch, coconut water is tolerated fairly well by humans. In fact, coconut water preserves teeth better than milk – something to keep in mind the next time a tooth gets accidentally knocked out!
In the 19th century, no one understood that people had different blood types. Blood transfusions often resulted in death, as the receivers immune system would attack the foreign, unmatched blood that was transfused.
The history of blood transfusion goes all the way back to the 1600’s, when William Harvey discovered the circulatory system. By 1658, Jan Swammerdam was viewing red blood cells through a microscope. The very first transfusions occurred in dogs, as the English physician Richard Lower demonstrated that a dog could be kept alive by transfusing blood from other dogs.
Unfortunately, the move to human transfusion was quite tricky. As there was no understanding of blood groups, blood transfusions were extremely risky. Sometimes they were successful: in 1818 James Blundell managed to accomplish the first successful human blood transfusion, and saved a woman hemorrhaging from childbirth. Other people, however, simply went into shock and died after blood transfusions.
Some scientists attempted to prevent the adverse reactions to blood transfusions by transfusing blood substitutes. The transfusion of cow’s milk was attempted in 1854 in Canada, during a cholera epidemic. Drs. Bovell and Edwin Hodder started intravenous transfusions of milk in the belief that the fat molecules in milk could be transformed into white blood cells, and that white blood cells were an immature version of red blood cells. This belief was erroneous, of course, but they had success with one sick man who responded favorably to the transfusions. Two other patients, however, died after milk was transfused into their veins.
These experiments were discontinued in Canada shortly after the cholera epidemic, but were revived in New York City a few years later. Using goat’s milk this time, Dr. Joseph Howe transfused patients suffering from terminal tuberculosis. The patients all demonstrated nystagmus (shaking eye movements) and chest pain, and all of the patients died a few hours after transfusion.
Despite the lack of obvious benefit, milk transfusions continued in the late 1880’s, as the use of blood was discouraged since it had a tendency to coagulate. As more patients died from milk transfusions, the practice fell out of favor. In the 1880’s, isotonic saline solution was invented, and the use of milk fell entirely out of favor in favor of the new, safe saline solution. The revival of blood transfusions would have to wait for the 20th century, when a new era of microbiology ushered in the understanding of various blood groups and compatibility.
In 1901, an Austrian doctor named Karl Landsteiner recognized the three basic blood groups – blood was first cross matched in 1907. Blood storage was still a problem during the early days of blood transfusion – while the compatibility issues had been resolved, blood still had a tendency to clot during storage. Anticoagulants like sodium citrate were developed in the year 1914, allowing blood storage for an extended period of time. The discovery of Rh factor in 1940 allowed doctors to completely understand the compatibility issues among blood donors and recipients, and the American government started its first national blood collection program shortly thereafter.
A traumatic car accident has occurred, and a severely injured patient is rushed to the emergency room. As the patient lies bleeding, doctors scurry to take a sample of the patient’s blood and have it sent away to be typed and cross-matched.
In the laboratory, a technician applies the blood to a special card, which contains antibodies to the A and B blood groups. If the patient’s blood clumps around the A antibody, this means they have the B antigen and it is attacking the A antibody. If the patient’s blood clumps around the B antibody, then the patient has the A blood type. If the patient’s blood clumps around both the A and the B antibody, they have the O blood type, and if the patient’s blood doesn’t react to either the A or B antibodies, then he or she has the AB blood type.
In the case of our patient, the blood clumps around both the A and the B antibodies. The patient has the O blood type. An Rh test is also performed, and our patient is positive for this protein.
As determined by this test, the patient needs a transfusion of type O+ or O- blood. The blood bank releases type O+ blood for use, and the patient is then cross-matched to be sure there are no adverse reactions.
The sample of the O+ blood is taken from the blood bank and mixed with the patient’s blood in a test tube. The sample is watched for any adverse reaction, and if no clumping is noted, the blood is safe for the patient to use. The sample demonstrates no reaction with our patient’s blood, so the bag of donated, O+ blood is rushed down to the waiting patient. As blood is transfused, the patient’s vital signs improve.
The AB+ blood type is known as the Universal Receiver: an individual with this blood type can receive any other blood type without reaction.
The AB- blood type may receive blood types A-, B-, or O-; any transfused blood must be Rh negative to avoid reaction.
The A+ blood type may receive blood types A+, A- , O+, or O-.
The A- blood type may receive blood types A- and O-.
The B+ blood type may receive blood types B+, B-, O+, or O-.
The B- blood type may receive blood types B- or O-.
The O+ blood type may receive blood types O+ or O-.
The O- blood type may only receive the O- blood type. People with O- blood are known as Universal Donors, as their blood will not cause a reaction with any other blood type when donated, since the blood lacks all surface antigens and will not provoke an immune system attack in the receiver.
For most people, blood type is of little consequence in life. Sometimes, however, a woman is Rh negative and becomes pregnant with a baby who is Rh positive. If this is the first pregnancy, the baby is usually fine because the mother’s blood doesn’t mix with the baby’s during the gestational period. Sometimes, however, the baby’s and mother’s blood mixes during delivery. The mother’s immune system then begins to mount a defense against the foreign protein.
When the mother gets pregnant for the second time with an Rh positive baby, the risks are much higher. In this case, the mother’s immune system may react to the foreign Rh protein carried by the baby. When this happens, the mother’s immune system attacks the baby’s red blood cells, causing them to rupture. The baby develops a form of hemolytic anemia, which can be fatal.
To prevent harm to the baby, the mother can be given injections of Rh immune-globulin. The Rh immune globulin is an antibody for the Rh factor: if any of the baby’s blood has made its way into the mother’s system, the Rh immune-globulin binds to the infant’s blood cells. These “borrowed” antibodies will prevent the mother’s immune system from producing her own.
If a mother demonstrates high levels of Rh antibodies in her blood system, the baby is carefully monitored. If the baby shows signs of distress, a procedure known as an exchange transfusion is sometimes performed to replenish the infant’s blood supply.
Blood types A and B are co-dominant, so if the father has blood type AA and the mother has blood type BB, the child will have a blood type of AB.
Blood Type O is recessive, so a child will only have this blood type if he or she gets two O blood type genes from his or her parents. If both parents are blood type O, all of the children in the family will have the O blood type. Another way this can happen is if the parents are heterozygous for the O allele: this means the mother may be blood type A, but her genotype (the genes she carries) are really AO. In this case, she expresses the A blood antigen, but she also has a gene for the O blood type. If she marries another heterozygote AO carrier, there is a chance that one of their children would inherit both O genes and then have the O blood type. The chance of this family having a child with the O blood type is 25% – there is a 50% chance they would have a child with the AO genotype (which would have the A blood type) and a 25% chance they would have a child with the AA genotype (A blood type).
Blood type A is dominant over blood type O, so anyone who has one A gene will have the A blood type, even if they carry one type O gene.
Blood type B is dominant over blood type O, so anyone who has one B gene will have the B blood type, even if they carry one type O gene.
Rh factor is dominant, so a parents who are have two alleles for Rh factor will have children who are Rh positive. If the parents are heterozygous (having one Rh factor allele and one Rh negative allele), they have a 25% chance of having an Rh negative child. If both parents are Rh negative, all of their children will be Rh negative.