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Chimeras, Mosaics, and Other Fun Stuff

A friend who is a chemist told me about a recent discovery that fascinated me. Apparently, when a woman has a baby, not all of that baby's cells leave her body when the baby is born. He said fetal cells have been found living and growing in different sites in the mother's body. Is this true? Would that make her a mosaic? And what about babies that are miscarried--does the mom still carry cells from that baby in her body?

-A college instructor from Washington

September 10, 2010

What you describe is called fetal microchimerism (FMc). I had never heard of it before I read your question, but it turns out it may be pretty common. It could even play a role in how the woman's body deals with some diseases.

FMc happens when a very small number of cells from a fetus are found in the mother's body. It was discovered when scientists found bits of male DNA in some mothers' blood. Since that DNA didn't come from the women's cells, one likely source was fetal cells. More tests showed that the DNA matched the DNA of the mothers' sons.

FMc is different from being mosaic. Both involve two different sets of cells in one person's body. The difference is in where the cells came from.

In mosaics, both sets come from the same fertilized egg. The two sets of cells in chimeras come from two different sources. You can read more about the difference here.

Most women experience FMc during pregnancy. In many cases, most of the fetal cells are quickly cleared from the bloodstream afterward. But some women have small numbers of fetal cells in their blood for months or even years after being pregnant. Long-lasting FMc is even more common in women who have had miscarriages.

Scientists were as surprised as you were that this sort of thing could happen. It was so startling because for a long time the placenta was considered a perfect barrier. Nothing from mom was supposed to get in and nothing from the fetus was supposed to get out.

Doctors thought that this strict barrier was to protect the fetus from the mother's immune system. Since the fetus is "foreign," the reasoning went, the mother's body would reject it if their cells ever mixed. But clearly, the barrier isn't perfect, since FMc does exist!

Scientists don't yet know why FMc happens. But as you might guess, they have some theories.

They're pretty sure they know how FMc cells stay around, though. Fetal cells can look a lot like mom's (though they're not identical). Sometimes they may be alike enough that the woman's immune system lets them hang around. And these cells may have consequences for the woman's health.

The FMc cells might be a part of what causes autoimmune diseases. But they also might help fight cancer. Both the ability of FMc cells to hang around and these health effects happen through the immune system.

HLA: ID passes for cells

An important job of our immune system is to tell invading cells from our own cells. Foreign cells like disease-causing bacteria are identified and destroyed. Our own cells are identified and ignored.

So how does our body do this? With genes!

Everyone has 22 genes (11 pairs) for something called HLA. And each of those genes comes in several versions.

Picture 11 boxes. Each one has a mixture of colored lights. Some boxes have less than 10 different colors, and some have more than a thousand.

Each box represents one HLA gene and the colored lights are all the different versions of that gene that exist in the human population. Every person gets two lights from each box (since we all have two sets of genes, one set from each parent). Your immune system identifies your own cells by the combination of these lights.

Of course, our actual cells don't have lights. Instead, HLA genes are responsible for making proteins that decorate the outside of your cells. The combination of these proteins is like an identification code or signature. It marks the cells as yours.

As you can imagine, everyone has a pretty unique set of HLA genes. But children have a similar set compared to their mom -- they share at least half of them. And some kids share even more.

People of similar ethnicities tend to share more HLA versions than people with different histories. If both parents have similar backgrounds, they might already have some HLA versions in common. Their child will still get 11 HLA genes from each parent, but some of them might be the same version.

So it's possible for the child to have more than 50% similarity to his or her mother. And that may be why the FMc cells survive in the woman -- they are similar enough to mom's to escape detection.

This is just one theory and scientists don't know yet if it's the right one. But they are starting to figure out that FMc cells might be important players in some human diseases.

FMc and disease

FMc cells are often (maybe always) white blood cells. White blood cells are key players in fighting disease. So the FMc cells might play a role in how diseases happen in the mother.

White blood cells are the police that check cells for the right HLA code. But what if the white blood cells inside your body are themselves foreign? Sometimes they might be helpful, but they might also be harmful too.

Autoimmune diseases are one case where FMc cells might be harmful. An autoimmune disease happens when a person's immune system gets confused and attacks his or her own cells.

Scientists discovered that women with autoimmune diseases are more likely than other women to have FMc. One theory is that maybe it's actually FMc cells and not the woman's own that are doing the attacking in these cases. It could be that the disease happens because the FMc white blood cells recognize the mother's cells as foreign and go on the attack.

Of course, that's just one idea. And we definitely need to be careful! Just because women with autoimmune diseases are more likely to have FMc doesn't mean we know that FMc can cause autoimmunity. It could go the other way around or there could be a third factor that causes both.


FMc cells may
be double-
edged swords.

In fact, women who have been pregnant aren't the only people who get autoimmune diseases. Women who have never been pregnant can get them, and men and children as well. So we know that FMc definitely isn't the only factor in developing autoimmune diseases.

As I said before, these FMc white blood cells aren't all bad -- they may be helpful sometimes too. For instance, they could help fight off cancer.

Cancers happen when normal body cells grow out of control. Since they still have the right HLA molecules, it's hard for the immune system to know to attack them.

Scientists know that FMc is less common in women with cancer than in women who do not have cancer. Again, this observation doesn't prove that FMc is the cause of lower cancer rates. But it does lead to some interesting theories.

One idea is that the fact that FMc cells are foreign might come in handy in fighting cancer. Since the mother's immune system won't see the cancer cells as a threat, the foreign soldiers (FMc white blood cells) might be the only ones that will be able to fight the cancer.

So what I'm trying to say is that FMc isn't a curse. FMc doesn't 100% guarantee someone will get an autoimmune disease and it might even help their body fight cancer!

Of course, as is common for science, there's certainly a lot more studying that needs to happen before scientists know anything for sure. But what a fascinating phenomenon to study!


Robin Trujillo


Babies may leave
some cells behind
after they're born.


The combination of
HLA on you get from
mom and dad is like
an ID tag.