The most common blood group is O, while the most uncommon blood group is AB. The most common blood type is O positive, while the most uncommon blood type is AB negative.
There are eight different blood types divided into four groups that show up in human beings. The groups are O, A, B and AB and the type is specified by either a positive or a negative indicator. The second most common group is A, and the third most common group is B. Blood type is an inherited trait and cannot be changed during the course of a lifetime. Blood received from a donor in a transfusion must be the same blood type as that of the recipient.