Does HLA sharing cause miscarriages ?

There are many myths regarding the causes and treatment of recurrent miscarriages. This article helps to explode the ones around HLA sharing as a cause of recurrent pregnancy loss.

I recently heard from one of my friend who is going through IVF to have her child. In the course of her treatment , she went through two pregnancy losses, one at 12 weeks and one at 5 weeks. As usual, a battery of tests were performed on her by her IVF specialist to find out where the fault lay- why did the miscarriage happen ? Of the several tests performed on her, two tests came back positive. She was told her NK cells in uterus are elevated ; and that her HLA haplotype (what the heck is that?) was very similar to her husband's. On seeing her results, her doctor told her that it was a waste of time and energy in trying to get pregnant via IVF. He told her that she would never succeed in carrying a baby to term ; and that she needs a surrogate to carry the baby.

This was a great shock for a woman who was already going through hell in order to have a baby ? Going through IVF is a physically and emotionally draining pursuit. Facing a miscarriage after successful IVF is heart-wrenching. In this situation, how will you react when you are told that there are some killer cells in your uterus which are waiting to devour your precious embryo ? How would you feel when a doctor tells you that you share so much in common with your husband that you cannot carry a baby to term ? Normally, when someone tells you that you have something in common with your partner , you feel happy , but now there is a person telling you that too much sharing means that your joy of having your baby is at stake.

These kinds of diagnoses steal your confidence and the trust you have in your reproductive system and your ability to carry a baby!) . They make you miserable, as you do not know do what to do next. I am sure many of us are completely clueless about what HLA is and what natural killer cells (scary name !) are - I was , even though I am a biologist In fact I bet that most doctors who perform such complicated tests on you will be unable to explain to you what HLA is ; and how it is connected to miscarriages !If you insist on knowing more, you will be provided with a pamphlet which is full of Greek and Latin.

You wonder if there are any treatment options which will help you to have your baby; and you also start doubting - do I really need to treat this condition - can any of this be true. When you can't analyze the situation rationally and make a well-informed decision ; and when you have to rely on your doctor for making decisions about your further treatment options (especially when the treatment may cost you an arm and leg , and you are uncertain if the doctor himself understands what is happening) , you are sure to feel handicapped, angry and scared !

This article is intended to explain in simple terms what HLA is all about . Does HLA sharing really cause pregnancy losses ?Do the treatments employed to treat such a diagnosis help patients to carry their baby to term ? Do you really need to treat such a diagnosis ?

Let me start with a little bit of scientific background. Don't let your eye glaze over - stay with me please - it's an interesting journey and you'll enjoy the ride. I'll try to make it as painless as possible !

What is HLA ?
HLA stands for Human Leucocyte Antigen. They are a group of proteins which are expressed on the surface of almost all the cells in our body. HLA is also called as Major Histocompatibility Complex (MHC). The genes (stretches of DNA ) which encode for HLA proteins are present in chromosome 6 (we have 46 chromosomes- 23 pairs !).

HLA genes are classified into class I, class II and class III. Class I genes encode for HLA- A, HLA- B and HLA- C proteins. These proteins are present in most cells of our body. Class I region also contains genes which give rise to HLA-E, HLA-F, HLA-G proteins. Class II genes encode HLA-DP, HLA-DQ and HLA-DR proteins. These proteins are present mainly in our immune system cells like B-lymphocytes, macrophages, T-lymphocytes etc. Class III genes encode components of our complement system (a group of proteins which function in our immune system)

HLA genes are highly variable

The genes that code for HLA proteins are highly variable. Thus, the HLA-A gene is not the same in everyone. They carry minute differences in them , as a result of which they produce proteins which are very different too. The HLA-A gene contains 2,188 variants and these are called alleles of HLA-A. There are 1,571 different varieties of HLA-A protein produced in humans (not all gene variants end up producing a different protein; hence the number of proteins is not equal to the number of gene variants). Each of us carry a pair of HLA-A gene and have only two variants (alleles) of the HLA-A gene. These 2 alleles can be any of the 2,188 variants present. For example if you assign each allele of HLA-A gene with a numerical name from 1-2,188, then I can have HLA-A allele HLA-A* 21 and HLA-A *892 (usually represented like this : HLA-A*21.892) and my husband might have HLA-A*01 and HLA-A*1076 ( the scientific shorthand for this is HLA-A* 01,1076) This is the same for all other HLA genes as well - HLA-B, HLA-C, HLA-DP, HLA-DQ or HLA-DR. If you understand this , you will be able to make sense of your HLA typing result.

What is the function of HLA in human body ?

There are more microbes in this world than humans and there is a constant war going on between them. Microbes try to attack us all the time and we have to fight them in order to prevent diseases. Just like police searching for thieves, special cells are constantly patrolling our body in search of harmful microbes like bacteria and viruses which can hide within us. If a thief hides within a house, someone in the house should tell the police that the thief is hiding inside , so the police can trach him down. Likewise, when a microorganism enters and hides in our body , there should be some alerting system which should tell our immune cells ( the cells in our body which fight against microbes and kill them) that a particular cell is infected with a microbe and that they should destroy the microbe or the entire cell , in order to protect the other cells of our body. This is the job of the HLA molecules! HLA proteins help our immune system to identify microbes and thus help to kill them.

Let's look at this in more detail. When a cell is infected with a virus, some viruses are killed and broken down into pieces by the infected cell's machinery . A part of the virus (a protein molecule also called an antigen ) is presented by the HLA class I molecule (which is present on the cell's surface ) to the cytotoxic T cells of the body's immune system . The T cell has the power to destroy an infected cell and this is how the immune system can identify which particular cell is infected , allowing the immune system to selectively destroy that cell , along with the invading virus.

Similarly, when bacteria enter our body , special cells of our immune system engulf the bacteria, digest them and present the bacterial protein to helper T cells (immune cells which coax other immune cells to produce antibodies which will eventually destroy the bacteria) , with the help of the HLA class II molecule.

This means HLA helps in telling our body's immune system that our body is under attack by microbes, and it helps in recruiting and activating the immune cells to fight this attack . Any microbial protein will be identified by our immune system only when that microbial antigen is presented to the immune cells by the HLA proteins of our own body !

This is the reason why HLA genes have so many variants. This large variety of HLA proteins helps in presenting different microbial antigens to the immune cells more effectively. If all humans carried the same HLA molecules, some new virus or bacteria which cannot be presented effectively to our immune cells would spread quickly and destroy the entire human race ! The more the variety in HLA proteins , the greater the chances that the human race will survive a microbial attack ! There are currently 8,949 HLA gene variants identified in humans ; and this number is rapidly expanding too !

HLA is responsible for the immune rejection or immune acceptance of transplanted organs

Every human carries his own set of HLA proteins in his cells, half of which come from the mother and half from the father. If my kidney fails to function , I need a kidney transplant. However, I cannot get a kidney from anybody - I need a kidney donor whose HLA proteins match mine. Foreign (non-self) HLA molecules are readily recognized by my immune cells. Any foreign HLA molecule identified by the immune cells alerts my immune system to mount an attack against that particular tissue or organ, because anything foreign is considered to be dangerous by our immune system. This is why the same immune system which protects our body from harmful microorganisms also prevents us from getting a potentially life-saving organ donation from a stranger. In order for any transplant (liver, skin, kidney transplant etc) to be successful , the HLA molecules present in the transplant recipient must be similar to the donor. This is why HLA typing is done before any transplantation procedure in order to identify the perfect donor. This is why HLA is also known as Major Histocompatibility (Histo = tissue) complex. Tissue compatibility during transplantation is decided based on HLA.

Since HLA genes are highly variable (remember, there are 8,949 HLA gene variants!) , it is very difficult to find a donor who matches the recipient. Only identical ( monozygotic ) twins have identical HLA proteins in their cells. Your sibling has only a 1 in 4 chance of being an identical HLA match for you.

How does HLA comes into play in reproductive biology ?

A human fetus receives half of the genes from its mother and half from its father. This means a fetus expresses HLA proteins which are different from its mother ( because 50% of HLA proteins in a fetus come from the father ). If a fetus is not an identical HLA match to its mother , then why doesn't the mother's immune system reject the fetus during her pregnancy ? After all, if the son tried to donate a kidney to his mother after birth, there's a high probability that this would be rejected ! (Remember, HLA mismatch is the strongest predictor of transplant rejection).

If a HLA mismatched organ is rejected by the recipient's immune system, then you would quite logically assume that if your husband carries the same HLA molecules as you do, then the chance of your immune system attacking your fetus is less , because the fetus carries the identical HLA molecules you have. But reproductive immunology theory says exactly the opposite !

How is an HLA incompatible fetus carried by the mother ?
A scientist named Wegmann in 1984 proposed a theory called immunotrophic theory to explain the paradox of how the maternal immune system accepts a fetus which is not genetically identical to hers. The theory states that recognition of the foreign antigen ( the paternal HLA) by the mother's immune system is good for the fetus , because it causes the maternal immune system to produce protective cytokines ( such as G-CSF),which enhance implantation, promote the growth of trophoblastic cells and help in sustaining pregnancy. Reproductive immunologists suggest that recognition of foreign fetal antigens (mainly paternal HLA) by the mother's immune system is necessary for eliciting a protective immune response ( such as the production of protective blocking antibodies , which protect the fetus from immune rejection), and this actually helps in the survival and development of fetus.

If HLA incompatibility between the mother and fetus helps in embryo implantation and growth, then won't HLA compatibility between mother and fetus lead to harmful pregnancy outcomes ? This is the question which prompted scientists to study the association between HLA sharing between partners and recurrent pregnancy loss.

HLA sharing between partners and fetal loss - what do the scientific studies say ?

This is a highly controversial topic among the scientific community ! One group states that HLA similarity (HLA compatibility) between the mother and fetus disrupts the proper cytokine signals and hence the mother's immune cells are activated abnormally, leading ultimately to the death of the fetus. (PMID: 7762569PMID: 8296846 PMID: 8207709PMID: 1729895PMID: 3860403 PMID: 10336015PMID: 1771630 )

The other group refutes this hypothesis and argues that their scientific findings do not support the notion that HLA compatibility between mother and fetus (HLA sharing between partners) can cause recurrent spontaneous abortion.(PMID: 2658473PMID: 3477351PMID: 15713211PMID: 20492598PMID: 11530859PMID: 8579756PMID: 1534444PMID: 20605049PMID: 20566486PMID: 8839135PMID: 1358093PMID: 3245544)

Studies which support the hypothesis that HLA sharing between partners causes fetal loss have severe limitations in their experimental design

Imagine you want to ascertain whether coffee drinking leads to heart attacks in humans. You select 100 people who suffered a heart attack and 100 who didn't. Then you ask how many in the heart attack group drink coffee , and compare this with the number of coffee drinkers in the non-heart attack group. If you find more coffee drinkers in the heart attack group , you conclude that drinking coffee leads to heart attack. This kind of study design ( called a retrospective study) is extremely flawed - 100 people are not enough to study the entire population. If you conduct such a study 10 times , the probability of finding more coffee drinkers in heart attack group simply by chance is 50 %. This means that if 10 scientists conduct such a study, 5 will report that there is an association between coffee drinking and heart attacks and the other 5 will say that there is no association ! This is what exactly happened in studies which are designed to find whether HLA sharing between partners causes pregnancy loss !

A better way to do the study would be to follow 10,000 people who drink coffee ; and 10,000 people who do not drink , to see how many in each group suffer from a heart attack over a period of time. This is called a prospective study - but is much more complex and expensive to perform !

About 3 % of women suffer from recurrent spontaneous abortion (RSA). RSA is defined as 3 or more miscarriages that occur before 20 weeks of gestation. Scientists who studied the association between HLA sharing and fetal loss, studied couples who had had RSA ; and compared their degree of HLA sharing with normal fertile couples who had had no pregnancy losses. If they found more HLA sharing between partners in the RSA group , they concluded that RSA is caused because of HLA sharing ! Because the sample size used for the studies is very small , they have severe limitations in their design, which is why you should take their results with a generous pinch of salt. Keep in mind that they are many other genes which are involved in a baby's development , and defects in these can cause miscarriage too. The studies conducted didn't ( and can't!) take into account all these confounding factors.

So, if there are studies which both support and refute the hypothesis , how do I interpret the scientific data ?

Here is where we should use our common sense ! We have so many inbreeding communities in the real world. Endogamy (practice of marrying within a specific ethnic group, class, or social group) is still common in many cultures and ethnic groups (Middle East, Assyrians, Jews, Yazidi Kurds, Muslims, Knanaya Christians, Hindus). If a population is inbred , then there is a high chance that partners share their HLA haplotype. Do such inbred communities suffer from RSA and infertility ? For example, India has several communities where inbreeding ( marriage between uncle and niece ; and marriage between first cousins) is very common. This study (PMID: 11874619) conducted on an Indian community which practices consanguineous marriages failed to identify any overall adverse effect of inbreeding on fertility. The results of many such studies showed that consanguineous couples actually had more children ! Your first cousin. In India, marriages with first cousins ( who have a 1 in 16 chance of being HLA identical ) have been practiced for centuries in many communities ; and these communities have flourished. If such marriages affected human reproduction adversely, the communities would have stopped following practices which were harmful.

Some more interesting information about HLA

When HLA testing is performed in the lab on you and your partner, they mostly check for compatibility between HLA-A, HLA-B, HLA-C, HLA-DQ etc. Do you know that these HLAs are not even expressed in the trophoblastic cells that come in contact with the mother's immune system ? The only HLA type that is expressed on trophoblast cells is HLA-G ! Doesn't this simple truth tell us many things ?

Should I consider paternal lymphocyte immunization therapy for treating the HLA sharing between me and my partner ?

Paternal lymphocyte immunotherapy has been used for treating RSA patients. It was hypothesized (one of the many existing hypothesis!) that the inability to build protective antibodies against fetal HLA (because of HLA similarity between mother and fetus as a result of the HLA sharing between the parents ) led to overactive NK cells and other immune cells , which killed the embryo and led to fetal loss. These antibodies are produced in a normal pregnancy, where they control and regulate the overactive NK cells , and protect the pregnancy from attack by them.

In Lymphocyte Immunization Therapy (LIT) your partner's lymphocytes are isolated from his blood and injected under your skin , so that you can mount an effective immune response against the MHC molecules which his lymphocytes carry. The immune cells which are produced in your body against the MHC molecules are thought to protect the pregnancy.

These publication (PMID: 16625529) (PMID: 22349103) states that such unproven therapies do not provide any significant beneficial effect than placebo.

There are also studies which show that paternal lymphocyte therapy can cause adverse effect to mother and fetus ! PMID: 21062364PMID: 1453393PMID: 10708244

After comparing the evidence for and against the concept of HLA sharing and recurrent spontaneous abortion , I am convinced that the diagnosis of HLA sharing between partners , and the paternal lymphocyte therapy which is used as treatment has little scientific validity ! It appears to be more of a pseudoscience than a proven scientific theory to be taken too seriously.

Should I undergo therapy for HLA sharing ?
If you suffer from recurrent abortions and do not know what to do next , you may be desperate enough to try every possible therapy under the sun before giving up . You maybe willing to be an experimental guinea pig, since there is very little modern medicine can offer in the form of treatment for this distressing disorder. You may be willing to undergo unproven therapies , and this is quite understandable. Even if you do decide to do so, please understand that such therapies have only anecdotal success stories , and are not a magical cure for your condition ! Over 50% of women who have had 3 or more miscarriages eventually achieve a successful pregnancy without any medical intervention, with the help of TLC - tender loving care ! Remember, HLA sharing between partners causes miscarriage is a theory which not has not proven. Please question your doctor if he pressurizes you to undergo treatment which is unproven and whose safety is not tested properly.

I wish doctors would stop demoralizing infertile women with scary unproven theories and therapies !

This is an excerpt from our forthcoming, book, The Expert Patient's Guide to IVF. This being authored by our expert patient, Manju and me.

You can email Manju at

Her blog is at

Authored by : Dr Aniruddha Malpani, MD and reviewed by Dr Anjali Malpani.