The Australian scientist Karl Landsteiner made the classification of the blood groups according to the differences he observed. By mere looking and by taste every man’s blood looks same, but the differences can only be observed by visualizing it under the microscope. For this the Karl Landsteiner was awarded the Nobel Prize.
By visualizing the blood under the microscope two different chemical molecules (Antigen) were identified. One molecule is given name as the “A” molecule (Antigen) and the other molecule is named as the “B” (Antigen) molecule. The blood with “A” molecule (Antigen) on its surface is called as the type A blood. Same way the blood with the “B” molecule (Antigen) on it is said to be type B blood.
And if the blood is having both the molecules (Antigen) on its surface then the blood is said to be blood of AB type.
A person with AB blood type can donate blood to only AB type of Blood.
A person with AB blood type can receive blood from any blood type.
Hence, AB blood type is called as the Universal Receiver.
And the O blood type is called as the Universal Donor