The eight blood types are A, B, AB and O, each of which can occur as a positive or negative type. The most common blood type is O positive, while the rarest blood type is AB negative.
Individuals with the blood type AB positive are universal receivers, meaning their immune system can accept any blood donation. Individuals with the blood type O negative are universal donors, meaning their blood donation can be given to someone with any blood type.
The four different blood groups, A, B, AB and O were discovered in 1901 by Karl Landsteiner. His work earned him the Nobel Prize in 1930. Landsteiner was also involved in the discovery of the Rh factor blood grouping system. In a particular blood group, the Rh antigen is either present or not, and this determines whether or not a blood type is positive or negative. As of 2015, about 85 percent of Americans have Rh positive blood.
Blood type is passed down genetically. Determining an individual's blood type can be done through ABO typing or back typing. In ABO typing, doctors mix blood with serums containing type A and B blood antibodies. A doctor can tell what blood type someone is based on whether or not the blood cells stick together. In back testing, A and B cells are added to a blood sample, and blood type is determined based on how it clumps with a particular cell type.