An average person typically holds approximately 5 liters of blood. To stay alive, the body generally needs to have more than 2 liters of blood in the system. It can be life-threatening if more than 2 liters of blood is lost in the body. For this reason, simply donating blood can help save a life.
The classification of blood is based on the absence or presence of antigens, which are substances that initiate an immune response. Safe blood transfusions usually depend on careful and correct blood typing and crossmatching since some antigens can trigger the body's immune system to attack the donated blood.
Four major blood groups are determined by the absence or presence of two antigens (A and B) on the surface of RBCs (red blood cells):
- Group A - People who belong to blood group A have A antigens on the surface of their RBCs and B antibodies in their blood plasma.
- Group B - People who belong to blood group B have B antigens on the surface of their RBCs and A antibodies in their blood plasma.
- Group AB - People who belong to blood group AB have both A and B antigens on the surface of their RBCs but do not have both A or B antibodies in their blood plasma.
- Group O - People who belong to blood group O do not have A or B antigens on the surface of their RBCs but have both A and B antibodies in their blood plasma.
Rhesus (Rh) Blood Group System
There are also antigens on the surface of RBCs called Rh factor or Rh-positive (Rh+). People who do not have it are called Rh-negative (Rh-). Naturally, people who are Rh-negative do not have Rh antibodies in their blood plasma. However, people with Rh-negative blood can also develop Rh antibodies in their blood plasma if they receive blood from other people with Rh-positive blood since Rh antigens can trigger Rh antibody production. People with Rh-positive blood can receive blood from others who have Rh-negative blood with no problems.
Blood transfusion is the process of transferring blood through an intravenous (IV) line and into the blood vessels. A blood transfusion is usually done to replenish blood loss during surgery or in serious injuries.
Blood from donors is usually mixed with the blood of the recipient to confirm the compatibility of blood. This pretransfusion testing is called crossmatching. However, crossmatching cannot always be done, especially in emergency cases.
Transfusing non-matching blood groups usually result in a hemolytic transfusion reaction, which is a very dangerous reaction due to ABO or Rh blood group incompatibilities. However, there are also cases of allergic reactions in blood recipients even though they receive a matching blood group.
As previously mentioned, A, B, and O blood types are determined by the absence or presence of A and B proteins on the surface of RBCs. Those who have blood type A have A antigens, those with blood type B have the B antigen, those who have an AB blood type has both A and B antigens, and people with blood type O have no A and B antigens.
The Rh blood grouping is a separate grouping system, which involves a different type of protein called the D protein. People who are Rh-positive have the D protein, while those who are Rh-negative do not have it.
Since the proteins A, B, and D on the surface of red blood cells all function as antigens, they can trigger an immune response. However, people with blood type O-negative do not have all these antigens, so it is the least blood type to create an immune response in blood recipients, making them universal donors.
The blood of a universal donor can be given to any blood recipient since it does not have incompatible antigens. Aside from being a universal blood donor, people with blood type O-negative are also regarded as universal organ donors.
An individual who can accept blood transfusions from any blood type is regarded as a universal recipient. To explain this further, if a person has blood type AB, he or she can accept blood from donors who have AB, A, B, or O blood types.
However, since blood type AB is the rarest blood type, why is it also the universal recipient? The answer is usually in the antigens present in most of the blood types. People with blood type O are unique since they do not have any antigens on the surface of their red blood cells. Those who have blood type A have specific A antigens, and those who have blood type B also have specific B antigens. People who have blood type AB posses both A and B antigens on the surface of their RBCs.
Since A and B antigens exist in people with blood type AB, they will not reject any type of blood. Their immune system usually identifies other blood types as self instead of being foreign. Those who have blood type O do not have any antigens, so they are considered as the universal blood donors since no reactions would occur.
To simply put it, people with blood type AB possess all of the antigens, while those who have blood type O-negative do not have any antigens that would cause a transfusion reaction. Universal recipients are also able to accept organ transplants from people with any blood type.
A donor's blood type should ideally match the recipient's blood type. However, in some cases, in which there is no time for testing blood compatibility, a universal blood donor can donate blood to the recipient, especially when blood transfusion needs to be immediately performed or when there is a shortage or an unavailability of the recipient's blood type.
Type O-negative blood is usually short in supply since this blood type is often used during emergency blood transfusions. For this reason, the demand for type O-negative blood increases in hospitals and blood banks, which is why people who are universal donors are encouraged to generously donate their blood to help save the lives of other people.