Everyone’s blood seems to look the same, but when magnified beneath the lens
of scientific devices, there are some differences. The four main blood types
are A, B, AB, and O, but in the final analysis there are actually 8
distinct blood types.
A Rh-positive (34% of
the population)
A Rh-negative (6% of
the population)
B Rh-positive (9% of
the population)
B Rh-negative (2% of
the population)
AB Rh-positive (3% of
the population)
AB Rh-negative (1% of
the population)
O Rh-positive (38% of
the population)
O Rh-negative (7% of
the population)
(The above numbers are an approximation for the United
States population. Distribution may be
different among specific ethnic and/or racial groups.)
Early in the 20th century, Karl Landsteiner, an Austrian scientist who won a
Nobel Prize for his achievements, observed two distinct molecules present on
the surface of red blood cells. They are called antigens and are the
markers used to determine a person’s blood type. The two molecules were simply labeled
“A” and “B.”
Type A blood has A antigens
covering the red blood cells.
Type B blood has B antigens.
Type
AB blood has both A and B antigens.
Type O blood has neither.
Rh Antigen
There is also the Rh antigen which some people have and others don’t.
The blood cells that do have it are marked RhD positive. So “B+” means the
blood has B antigens and Rh antigens, and “B-“ follows suit with B antigens but
no Rh antigens. Approximately 84% of the population is Rh positive.
Blood Compatibility
This all matters if a person needs a blood transfusion (adding blood
to a person’s circulation, usually from another person), because in certain
combinations the blood cells will clump together. These clots cause potentially
fatal situations. In the ideal situation blood types are matched before the
transfusion takes place, but type O blood is often used in emergency situations
because it is usually accepted by all blood types.
Type A blood can be given to
type A or type AB.
Type B blood can be given to
type B or type AB.
Type
AB blood can be given only to another
type AB.
Type O can be given to all
other types.
Because type AB people can accept any other type, they are referred to as “universal
recipients.” Type O people are considered “universal donors.”