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Blood and the Body

Understanding Blood Types


Your genetic code determines your hair and eye color, whether you are right or left handed, and whether you can curl your tongue, but it also determines your blood type. There are two main blood groups, the ABO group and the Rh group. The ABO group is made up of A, B, AB, or O blood types, and the Rh group is either positive or negative. You blood groups help doctors know many things, like what kind of blood you’ll need in a blood transfusion or an organ donation, and if you or your baby will be in danger if you get pregnant.

How do you know?

So how does your body know what type of blood you have? All of your red blood cells have special markers called antigens on the surface that tell your body, “I’m your blood!”

ABO Blood Types

Type O blood has no antigens on the surface of the red blood cells and is the most common blood type all around the world. People who

 are type O can donate blood to anyone, but can only receive type O blood. Type A blood, which is more common in Central and Eastern Europe, has A antigens on its surface. People who are type A can donate to either type A or type AB people and can receive blood from type O or type A people.

Type B blood, which is more common in Asia, has B antigens on its surface. People who are type B can donate to either type B or type AB people and can receive blood from type O or type B people. Type AB blood is the least common, showing up mostly in Japan, China, and Korea. It has both A and B antigens on its surface. People with type AB blood can only donate to others with type AB blood, but can receive blood from anyone.

Rh Blood types

Rh (+) positive blood means that your red blood cells have D antigens on their surface. Rh (-) negative blood means that your red blood cells do not have D antigens on their surface.

Rh (+) blood is much more common than Rh (-) blood all over the world. In Caucasians, the ratio of Rh (+) to Rh (-) blood is approximately 4.5:1, in African Americans and Latino/Latina Americans the ratio is approximately 13:1, and in Asian Americans the ratio is approximately 49:1. People who are Rh (+) can donate blood to either Rh (-) or Rh (+) people, but can only receive blood from those people who are Rh (+). People who are Rh (-) can donate blood to only people who are Rh (-) but can receive blood from both Rh (+) and Rh (-) people.

Why does it matter?

When you receive a blood transfusion, someone else’s blood is being put into your body. If the new blood cells have different antigen surface markers on their surface that are different from yours, then your body will see them as invaders. Your immune system will create antibodies which will destroy the new blood. Your body’s reaction to mismatched blood can be very serious, even life-threatening. Routine typing and cross matching of blood types can typically prevent any transfusion complications. That is why your blood type is determined by both the ABO and Rh groups before you are given any kind of blood transfusion. Whether you are Rh (+) positive or Rh (-) negative can have a serious impact on the health of a newborn. The problem occurs when the mother is Rh (-) negative and father of the baby is Rh (+) positive which can result in a baby with Rh (+) positive blood. This can be dangerous for the health of the baby when it is born.

During pregnancy, fetal blood passes through the placenta to the mother. If the mother and the baby are Rh incompatible, the mother’s body will see the baby’s blood as an invader. Just like with blood transfusions, the mother will produce antibodies against the baby’ blood which will attack the fetal red blood cells. When the red blood cells are destroyed, the baby will have a limited supply of oxygen, called hemolytic anemia, and could die. Usually first born children are not at risk for complications due to Rh incompatibility because the mother’s body is just beginning to develop antibodies. However, when the second Rh (+) positive baby is developing, the antibodies are poised and ready to attack. In a mild case if no treatment is received, the newborn will have yellowing of the skin and eyes, low muscle tone, and will be sluggish and sleepy. In severe cases with no treatment received, the newborn could have severe damage, other serious related illnesses, or they could die.

It is important to receive prenatal care so that doctors can find the blood type of both you and the baby. If you are negative and your baby is positive, preventive treatment can be given during pregnancy. Knowing your blood type and the blood type of your partner can help you determine whether you are in danger of having pregnancy complications due to blood type. Antibody screening can show if a woman has developed antibodies to Rh (+) positive blood, and an injection of RhoGAM, an Rh immune globulin, can prevent the woman from producing antibodies that can harm her baby. RhoGAM is received by the mother twice; once in her second trimester and once a few days after delivery. The injections contain antibodies to the D antigen which will prevent the woman from producing the dangerous antibodies.


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Last Reviewed: May 01, 2012

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Director, Area Health Education Center
School of Medicine
Case Western Reserve University