Evolutionary Significance of Blood Types

[tursiopstruncatus]

I know that animals have different blood types just as people do, but why do they exist? There has to be some sort of advantage to having (or not having) certain proteins in one's blood. How did different blood types help species to survive?

[mith]

cholera resistance

[MichaelXY]

http://anthro.palomar.edu/blood/ABO_system.htm