The blood types and their global prevalence are A+ at 28.3 percent, A- at 3.5 percent, B+ at 20.6 percent, B- at 1.4 percent, AB+ at 5.1 percent, AB- at 0.5 percent, O+ at 36.4 percent and O- at 4.3 percent. The rarity of each type varies greatly by region.
Blood type is a trait controlled by genes, and it denotes the presence or absence of different chemical markers found in the blood of people with those genes. The letters "A" and "B" each designate a certain type of antigen found on the surface of red blood cells of people with those genes. Their presence depends on the particular allele of the same gene. Since humans have two copies of each gene, one from each parent, a person can produce both markers and have type AB blood. The letter "O" designates a third variation of this gene that does not produce antigens on the surface of blood cells.
The symbols "+" and "-" designate the presence or absence of Rh proteins in the blood. If a person has a blood type that lacks any one of these chemical markers, the person's immune system attacks any blood with those markers. This can cause dangerous clotting when a person receives the wrong blood type in a transfusion.