

Why Are There Different Blood Types?

Scientists believe the different types manifested as a result of early humansadapting 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 . . . . As a result, people with type O get less sick when they're infected with malaria . . . ."

How Are Blood Types Classified?




Antigens and Antibodies

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] the immune system[] will attack any donor red blood cells that contain antigens that differ from their self-antigens."

The attackers are found in blood: "Plasma [which] contains Anti-A and Anti-B antibodies depending upon blood group. . . . Antibodies are important molecules our immune system makes to help protect ourselves against foreign things such as bacteria and viruses [and] . . . can also be formed in response to different blood groups."


Rh Factor


Women who've been pregnant may remember the Rh blood test, which screens to detect anyincompatibility: "If [mother] is Rh-negative and . . . baby is Rh-positive, [mother's] body will react to the baby's blood as a foreign substance. [Mother's] body will create antibodies (proteins) against the baby's Rh-positive blood . . . . Rh incompatibility is more likely to cause problems in second or later pregnancies [when] Rh antibodies can cross the placenta and attack the baby's red blood cells . . . lead[ing] to hemolytic anemia in the baby."



What Are the Different Types?

Type A

The second most common blood type across all major ethnic/racial groups in the U.S., type A blood is characterized by the presence of the A antigen on its red blood cells, and the B antibody in its plasma.



Type B




Type AB

Uncommon, only 7.1% of Asians, 4.3% of African-Americans, 4% of Caucasians and 2.2% of Hispanics in the U.S. have the AB blood type. It is 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).



Type O



About 57% of Hispanics, 51% of African-Americans, 45% of Caucasians and 40% of Asians in the U.S. have type O blood.

What Happens if You're Given the Wrong Blood?

Very bad things, including an:

Acute hemolytic transfusion reaction[ which can] occur within 24 hours of the transfusion and often occur during [it]. Ominously, the patient may report a "feeling of impending doom". They may also complain of a burning sensation at the site of the infusion, together with chills, fever, and pain in the back and flanks . . . . The most sever reactions involve intravascular hemolysis; the donor RBCs [red blood cells] are destroyed by the recipient's antibodies while they are still inside blood vessels . . . . Hemoglobin is released . . . and excreted in the urine . . . turning [it] dark brown. . . . Bilirubin . . . usually secreted into bile by the liver, instead accumulates in the blood causing jaundice. Massive activation of complement can cause shock, as can the large amounts of tissue factor released by RBC [red blood cell] debris that triggers uncontrollable clotting cascade (disseminated intravascular coagulation).


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