Scientists believe our blood was typed as a result of our adaptation to infectious diseases. For example, malaria appears to be the main selective force behind type-O, which is more prevalent in Africa and other parts of the world that have high burdens of malaria, suggesting that blood type carries some sort of evolutionary advantage. In this particular case, cells infected with malaria don’t stick well to type-O or type-B blood cells and as a result, people with type-O get less sick when they’re infected with malaria.
Blood type are classified according to the presence or absence of two antigens – A and B – on the surface of red blood cells. It is also described as either positive or negative, depending on the presence or absence of the Rh factor.
Antigens, Antibodies and Rh Factor
Generally speaking, antigens are any substance to which the immune system can respond, and with regard to blood, the membrane of each red blood cell contains millions of antigens that are ignored by the immune system although are responsive to any antigens that differ from their self-antigens.
Antibodies are important molecules our immune system makes to help protect us against foreign things such as bacteria and viruses and can also be formed in response to different blood groups. They are found in blood plasma as Anti-A or Anti-B antibodies depending upon blood group.
Most people have a special protein on their blood cells called the Rh factor, a condition that makes them Rh positive and have a positive blood type. Others who lack Rh factor, are called Rh negative and have a negative blood type.
What Are the Different Types?
Type-A blood: characterized by the presence of the A-antigen on its red blood cells, and the B-antibody in its plasma. People with type-A are safe to donate blood to others with type-A blood, as well as those who are type-AB. In addition, people with type-A may also receive transfusions of type-O blood.
Type-B blood: characterized by the presence of the B-antigen on its red blood cells, and the A-antibody in its plasma. People with type-B blood may safely donate to others with the same type, as well as those with type-AB blood. In addition, those with type-B blood may safely be transfused with type O blood, as well.
Type-AB blood: characterized by having both A and B-antigens on its red blood cells, and neither antibody in its plasma (meaning there is nothing that will attack foreign blood). For this reason, those with AB-blood type are sometimes referred to as universal recipients because they can accept blood from anyone; however, because of the presence of both antigens on type AB-red blood cells, people with AB-blood can only donate it to others with the same type.
Type-O blood: characterized with having neither A nor B-antigens on its red blood cells, but both A and B-antibodies in its plasma. As a result, those with type-O blood can only receive this type in transfusions since its plasma antibodies would attack anything else; however, those with type-O can also donate blood to anyone else since type-O is free from all immune-system antagonizing antigens.
What blood type are you?