I know that a woman with type O blood can have a healthy baby with type B blood. Why doesn't the baby develop a "hemolytic" type disease? I understand hemolytic disease of the newborn is related to Rh factors and understand how that part of it works, but just don't understand why a similar type of situation doesn't occur when they baby is of a different ABO blood type?

-A curious adult from California

September 18, 2008

Actually, a similar thing can happen to moms that have an O blood type. The reason you may not have heard of it before is because it isn't very common. And it's usually not very serious.

It happens for the same reason that moms with Rh- blood can have problems. Basically, mom's immune system attacks the fetus' red blood cells.

This attack causes the fetus to lose many of its red blood cells. This can lead to anemia, jaundice, and even stillbirth. But it's usually less severe for O moms compared to Rh- moms.

To understand why, we first need to figure out why mom's immune system might attack her fetus' red blood cells. And then why it doesn't happen every time a mom's blood type is different from than of her fetus.

Why Mom's Immune System Might Attack

Our immune system helps to protect us from getting sick. Our bodies can tell the difference between things that are normal and foreign stuff, called antigens. These can be found on things like bacteria or the flu virus that gets passed around every year.

Sometimes, though, a fetus can make antigens that mom doesn't. Then mom's immune system thinks that part of the fetus is foreign. And so mom's body attacks it.

How can a fetus have different antigens from a mom? Because a baby gets genes from both mom and dad. And genes have the instructions for making antigens. So if dad has a different gene version from mom AND he passes that gene to his child, then the child will have an antigen that mom doesn't.

Usually for an antigen to cause problems, it needs to be on the outside of a cell. This is why blood type antigens like A, B, or Rh can be such a problem. Because they are on the outside of our red blood cells.

Blood Type Genes

Two of the most important blood antigens in humans are those in the ABO and the Rh systems. Like any antigen, the antigens in these systems come from genes.

The ABO system has a single gene. This gene comes in three different versions: A, B, and O. This is where the four blood types, A, B, AB, and O come from.

People with A blood type make the A antigen and people with B make B antigen. People with AB blood type make both antigens and, what is important here, people with O blood type make neither antigen.

The Rh system is a bit more complex genetically but Rh+ people make the Rh antigen. And Rh- people do not.

This stuff matters because sometimes an O mom can carry an A or B child. Or an Rh- mom can carry an Rh+ child. In these cases, the babies have different antigens from their moms.

This is because of dad's genes. A baby gets one gene version from its mom and another from its dad to make its blood type. So if a mom is type O, she'll give the baby an O version of the ABO gene. But if dad is not O, he could give the baby the gene version to make the A or B antigen.

This can also happen with the Rh antigen. If dad is Rh+, he can give that gene version to the baby so the baby can make the Rh antigen. Either way, the baby can then make antigens that mom doesn't have. Then mom's body can notice that something is different.

If mom's immune system figures out there is an antigen that she doesn't have, then it attacks the baby's cells. The key to answering your question is in learning how the immune system figures things out. And why it sometimes can't see a baby's different antigens.

How the Immune System Works

Your body recognizes foreign antigens using something called antibodies. When a foreign antigen enters the body, our immune system makes a specific antibody against it. These are like little soldiers that remember which enemies have recently attacked so it can fight them off when they come back.

So when mom's body attacks the fetus' red blood cells, it is because her body thinks that it needs to protect itself against them. This is because the fetus' red blood cells have something extra on them that the mom doesn't have. This is the antigen that makes her produce antibodies that will attack the red blood cells.

Imagine an Rh- mom who has an Rh+ fetus. What this means is that the fetus has an antigen that mom doesn't. And so if mom's immune system comes in contact with the Rh antigen, it will make antibodies against it.

The same would be true for any accessible antigen that the baby has that mom doesn't. Including the A and B antigens.

Now finally we are ready to learn why Rh hemolytic disease of the newborn (as it is called) tends to occur more often that the ABO kind. It turns out the not all antibodies are created equal.

Your body can make 5 different types of antibodies: IgA, IgD, IgE, IgM, and IgG. Each has a different function in your body. The two that are important here are IgM and IgG.

IgM antibodies are made right after being exposed to an antigen. This is your body's first line of defense (called a primary immune response).

IgG antibodies typically show up about a month after exposure to an antigen in your body. They are the secondary response (after IgM). More than half of the antibodies in your body are IgG!

Another key difference is that IgG antibodies can cross the placenta to make it to the fetus, but IgM antibodies cannot. This matters because the body tends to make IgG antibodies against the Rh antigen, but IgM antibodies against the A and B antigens.

Because of this, an Rh- mom has a bigger risk with an Rh+ baby than an O mom with an A and/or B baby. But I did say that O moms are affected sometimes. How does this happen?

There are a couple of ways. First, A and B antigens exist many places in nature, so people with an O blood type can produce IgM antibodies against them. Occcasionally, this exposure can also trigger IgG antibodies to be produced.

Another way to make IgG antibodies against A and B antigens is if blood from the fetus crosses the placenta into the mother. These antigens from the baby's red blood cells can trigger the production of IgG antibodies (if the child is type A, B, or AB). This is also how IgG antibodies are made against the Rh antigen.

Now why does this happen only in moms with O blood type? People with type A or B blood make only the IgM antibodies against the other type. It is very rare for them to make IgG antibodies against the other.

So without IgG antibodies, the red blood cells of the baby are safe! Only a portion of pregnancies have blood type incompatibilities between the mother and child. Even less of those are mothers that have the IgG antibodies that will attack the fetus' blood. This is what makes it so rare!

By Jamie Conklin, Stanford University

If a baby has different antigens on its blood cells, then mom's immune system might attack.

Your immune system attacks foreign things to protect you from getting sick.