Human blood may appear to be all the same substance to the naked eye, however under a microscope it is quite a different story. Depending on the different sugars and antigens that are present in your own blood, there are several different types that your blood can have. First discovered in 1901 by the American scientist and pathologist Dr Karl Landsteiner when researching the number of deaths resulting from attempted blood transfusions, the basic types of blood groups were identified as being A, B, AB, and O, and another system due to the presence of agglutinin in the blood or not called the Rh , which makes it positive or negative.
Type O+ is the most common and people of this group have none of the sugars present in their blood that the other groups have, meaning that they can donate their blood to all the other groups, but can only receive blood type O when they need a transfusion themselves. If there are transfusions between different groups then there is a massive level of nonacceptance by the recipient and this combined with them being in a weakened or injured state to need the transfusion in the first place usually results in death. There are of course screens for this so that it doesn't happen, although every now and then there are still cases of this nature that arise due to clerical or administrative errors.
On a basic level of difference there are certain antigens present in either the outside of the blood cells or inside the cell. Type O have both types of antigens inside the cell only, and none on the outside, which is why they can be given to other groups without fatal reaction. Type A has type A antigens on the outside of their cells and type B antigens on the inside, and vice versa for type B people. The rarest type of group which is AB have both antigens both on the outside and inside of their cells. The presence of another type of antigen determines whether we are positive or negative in our blood type.
Although type O blood is the most common, it has been theorized that these different blood types are evolutionary rather then by chance. For example although in the western cultures of Europe and America O is the most common, in some parts of Asia there is a higher prevalence of type A. This shows that the different antigens probably evolved as a way of combating certain diseases specifically present in certain geographical locations, and that the greater mixture of blood types today in general society is because of people from different countries interbreeding both in recent times and in times of invasion or wars fr example throughout history.
The basic spread of how the blood groups are prevalent is: