Basics of blood type

 


8% of our body weight is made of blood. It carries oxygen, nutrients, hormones and other substances around our body to keep us alive. We learnt from school that there are four blood type categories: A, B, AB and O. But what are the differences between these blood types and why can't we just donate blood to another person with a different blood type?

Antigens and antibodies

Blood type is identified by different antigens and antibodies contained in the blood. Antigens are protein molecules on the surface of red blood cells (RBC). Antibodies are proteins in plasma. They form a defence system in our body, which detect foreign substances like germs and alert the immune system to destroy them.

The ABO blood types

  • Type A: Contains type A antigen on RBC and anti-B antibody in plasma.
  • Type B: Contains type B antigen on RBC and anti-A antibody in plasma.
  • Type AB: Contains both type A & B antigens on RBC but no antibodies in plasma.
  • Type O: Has no type A nor B antigen on RBC but both anti-A & -B antibodies in plasma.

Blood transfusion with a different blood type can be fatal. When type B blood enters a type A body, the anti-B antibodies detect type B antigen on the RBC and trigger the immune system to attack type B cells. This is called an ABO incompatibility reaction. The most frequent reactions are fever, chills and body aches. More severe symptoms are shortness of breath, blood in urine and loss of consciousness. Most hospitals and blood banks have systems and procedures to reduce the chance of such a situation. 

The Rh system

Some people may contain another antigen called RhD on the RBC. If RhD is present, a person is RhD positive. Otherwise, the person is RhD negative. This means there are 8 blood groups: Type A +/-, type B +/-, type AB +/- and type O +/-. 

Universal donor and recipient

People who carry O- blood type are called universal donors. This is because they can donate blood to any blood type. There is no type A/B/Rh antigen on the RBC so antibodies in the blood receiver will not attack O- blood cells. It is used most often during emergencies.

People with type AB+ blood are called universal recipients because they can receive the blood of any type. They do not have anti-A/-B/-Rh antibodies in their blood. However, they can only donate blood to another AB+ person.

References:

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