Blood types are classified by the ABO system and the Rh factor, determined by antigens on red blood cells, with eight main blood types. Compatibility is crucial for blood transfusions, as individuals have antibodies against certain antigens; for instance, type AB can receive from all types, while type O is a universal donor. Blood transfusions are necessary for patients who lose blood due to injury or surgery, and blood from donors is rigorously tested before use.

2. Blood Types
If a doctor talks about your blood type, usually he or she is referring to two things: your
type in the ABO system and Rhesus (Rh) factor. Human blood type is determined by antigens on
the red blood cells. An antigen is a structure on the cell surface that causes a human immune
response reacts to if the structure is foreign to the person’s body. Consequently, blood type
match is of crucial importance. The donor’s blood type is identified at the Blood Centre, and the
patient’s blood type is determined before transfusion. Here is everything you need to know!
How do blood types work?
Logically, you inherit your blood type from a mix of your parents’ genes. There are eight main
blood types, organized through two combined systems. These systems are ABO (blood types A,
B, AB or O) and Rh type or group (positive or negative). Therefore, your blood type is a
combination of these two systems. For instance, the most common blood type is O positive and
the least common is AB negative.
The most important blood group system is ABO, in which your blood is classified as A, B, O or
AB. This is determined by two antigens on the red blood cells:
A — red blood cells have only the A antigen
B — red blood cells have only the B antigen
AB — red blood cells have the A and B antigens
O — neither A nor B antigen
If a person has A, B or O blood type, their plasma contains antibodies that destroy the antigens
that the person doesn’t have. Here is a good example: If you have blood type A, it’s important
that you don’t receive a B type transfusion, as you have antibodies that will destroy B antigens.
If you have blood type O, you have antibodies that will fight the A and B antigens.

3. If a person has blood type AB, they don’t have such antibodies, and they can accept
transfusions from all other blood types. Thus, AB blood type people can be termed universal
patients.
However, O Rh negative donors can be called universal donors, as red blood cells from such
donors can be used for transfusions for all patients.
The Rh factor
Each blood type is also grouped by its Rhesus factor, or Rh factor. Blood is either Rh positive
(Rh+) or Rh negative (Rh-). Statistics says that about 85% of Americans have Rh+ blood. Rhesus
refers to another type of antigen, or protein, on the surface of red blood cells. The name
Rhesus comes from Rhesus monkeys, in which the protein was first discovered.
Blood Transfusions
A blood transfusion is the transfer of blood from one person to another. Blood that is lost
through an injury, an illness or a surgery can be replaced through transfusion. Aside from
transferring blood as a whole, part of blood, such as red blood cells, platelets or plasma can also
be transferred to individuals. Donor blood is always tested for HIV, hepatitis, syphilis, West Nile
virus and other diseases before every transfusion. There are more than 9.5 million blood donors
in the United States and an estimated 5 million patients who receive blood annually, resulting
in a total of 14.6 million transfusions per year.
What is the Rarest Blood Type?
According to the American Red Cross the rarest is AB(-), present in 1% of the Caucasians, in
African Americans it is even rarer. B(-) and O(-) are also very rare, each accounting for less than
5% of the world's population. Some people with rare blood types bank their own blood in
advance of surgical procedures to ensure that blood is available to them which is always a
smart move.

4. Blood types are inherited. Like eye color, blood type is passed genetically 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.