Without blood, the human body would simply stop working. This essential fluid of life dispenses crucial nutrients throughout the body, exchanges oxygen and carbon dioxide, and carries our immune system’s ‘militia’ to stave off infections. But not all blood is equal, and in the event of a transfusion, mixing incompatible blood types can lead to death.
Distribution of blood types
There are 4 main blood groups: A, B, AB and O, of which group O is the most common. In the United States, the average distribution of blood types is as follows:
- O-positive: 38 percent
- O-negative: 7 percent
- A-positive: 34 percent
- A-negative: 6 percent
- B-positive: 9 percent
- B-negative: 2 percent
- AB-positive: 3 percent
- AB-negative: 1 percent
Different racial and ethnic groups typically see a different distribution. For instance, 45 percent of Caucasians are type O, but 51 percent of African-Americans and 57 percent of Hispanics are type O, according to the Red Cross. This Wikipedia page has the blood type distribution in every country.
Type O is the most demanded blood type in hospitals, both because it’s the most common and because O-negative blood is a universal donor type, meaning it is compatible with any blood type. Conversely, type AB-positive blood is called the universal recipient type because a person who has it can receive blood of any type.
How blood type is determined
Like eye color, blood type is genetically inherited from your parents. Whether your blood group is type A, B, AB or O is based on the blood types of your mother and father. For instance, if your mum is AB and your dad is A, you can expect to be A, B, or AB. If mum is AB and dad is O, the child will have an A or B blood type. When both parents are A, the child will have either O or A.
Blood is essentially made up of two types of blood cells (red and white), platelets, and a fluid called plasma. About half the blood (45%) is made up of blood cells, with the remaining 55% being plasma. Millions of blood cells are produced daily in the bone marrow, the soft spongy material that fills up bone cavities.
A person’s blood type is determined by proteins found on the surface of red blood cells called antigens. If antigen A is present in the red blood cells, then you have type A blood, while having B antigen present means you have type B blood. If both A and B are present, you have type AB blood. If neither antigen is present, you have type O blood.
Besides the ABO classification, there’s also another blood type grouping that involves Rhesus (Rh) factors. The name comes from the Rhesus monkeys, in which such proteins were first discovered. Rhesus factor D, which is the most important, is present in 85% of people, making them Rhesus positive. The remaining 15% are grouped Rhesus negative. The Rh grouping can be very important in some situations. For instance, a baby’s life can be endangered if it inherits a Rhesus positive blood type from the father while the mother is Rhesus negative — in such a situation, the mother can form antibodies against her own baby’s blood.
Blood groupings and transfusions
In order to safely perform a blood transfusion, it’s essential that a patient receives a blood type that is compatible with their own. If the blood type is incompatible, the red blood cells can clump together, producing clots that block blood vessels and cause death. Generally, for the ABO grouping, blood transfusions follow these rules:
- A person with type A can donate to a person with type A or AB.
- A person with type B blood can donate to a person with type B or AB.
- A person with AB type blood can only donate to a person with AB. However, a person with blood type AB can receive blood from anyone, being the universal recipient.
- A person with O type blood can donate to anyone, being the universal donor. This is because type O blood has no antigen on the surface of its red blood cells. However, people with type O blood can only receive type O.
People with Rh-positive blood can receive either positive or negative donations but those who have Rh-negative blood can only receive other Rh negative blood.
Doctors will test your blood before you are allowed to donate or receive blood. However, in the event of an emergency when the patient’s blood type is unknown, type O blood will be used.
It’s important to note that there are more than 600 other known antigens, the presence or absence of which creates “rare blood types.” The ABO grouping works just fine for most people, but in some rare cases, certain blood types may be unique to specific ethnic or racial groups. For instance, many patients with sickle cell disease require an African-American blood donation. That’s why it’s still ideal to match a blood donation type to its recipient exactly, accounting for both antigen types and Rh factor.