Amalgamation of blood from two human beings can lead to blood clumping or agglutination. The clumped red cells can crack and cause toxic reactions resulting in fatal consequences. In 1901, Austrian Karl Landsteiner discovered human blood groups that made the blood transfusions safe, previously the transfer of blood or blood components into a person’s blood stream resulted in many deaths.

Quantity of Blood in Human Body
Blood transports oxygen to various parts of the body. An adult human has about 4–6 liters of blood circulating in the body.
What are Antigens?
Antigen is a substance, usually a protein, on the surface of a cell or bacterium that stimulates the production of an antibody.
What are Antibodies?
A protein produced by B cells in the body in response to the presence of an antigen, e.g. a bacterium or virus. Antibodies are a primary form of immune response in resistance to disease and act by attaching itself to a foreign antigen and weakening or destroying it.
Blood Types
The most recognized and medically significant blood types are in the ABO group. There are four principal types having two antigens and two antibodies that are mostly responsible for the ABO types. The specific combination of these four components determines an individual’s type in most cases.
- A
- B
- AB
- O
Blood Groups
There are four major blood groups determined by the presence or absence of two antigens – A and B – on the surface of red blood cells:
- Group A. It has only the A antigen on red cells and B antibody in the plasma
- Group B. It has only the B antigen on red cells and A antibody in the plasma
- Group AB. It has both A and B antigens on red cells but neither A nor B antibody in the plasma
- Group O. It has neither A nor B antigens on red cells but both A and B antibody are in the plasma
Table below will further make the ABO Blood Grouping comprehensible

Rh Factor in Blood Group
In addition to the A and B antigens, there is a third antigen called the Rh factor, which can be either present (+) or absent ( – ). In general, Rh negative blood is given to Rh-negative patients, and Rh positive blood or Rh negative blood may be given to Rh positive patients.
- The universal red cell donor has Type O negative blood type.
- The universal plasma donor has Type AB positive blood type.
Inheritance of Blood Group
In 1910, scientists proved that parents inherit the RBCs antigens and that the A and B antigens are inherited co-dominantly over O.
Distribution of Blood Groups

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