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What is Reproductive Immunology?

What is reproductive immunology?

Reproductive immunology is a field of reproductive medicine that focuses on the interactions between the immune system and reproductive system. It studies the role of immune cells and immune molecules at different stages of reproduction: fertilization, embryo implantation, early-to-late pregnancy, and labor and delivery.i

Why is immune function relevant to reproductive function?  

The main function of the immune system is to protect the health of the host by distinguishing between “self” and “non-self” foreign molecules within the human body. “Non-self” entities include bacteria, viruses, and other organisms.ii

After conception, 50 percent of an embryo and its cells contain genes and proteins from the egg; the other half of the embryo contains genes and proteins from the sperm. During pregnancy, some immune activity will be suppressed so that the embryo will not be recognized by the pregnant woman's body as foreign (given that half of the embryo contains genetic material from another person). This is a process called maternal-fetal immune tolerance.iii

This paradoxical immune response to prevent rejection of the semi-foreign embryo (and, prior to fertilization, the sperm) is regulated by the varying levels of estrogen and progesterone expressed throughout the menstrual cycle. These two important sex hormones coordinate the presence of immune cells (e.g., white blood cells) and the secretion of immune molecules (cytokines and chemokines) to create a balanced immune environment in the uterus.iv  

The process of embryo implantation itself involves an active immune response, including a degree of inflammation. During the window of implantation, which typically falls on days 19-23 of a natural menstrual cycle and lasts about 48 hours, several different white blood cells, including T cells and natural killer cells, are present in the endometrium (inside lining of the uterus). If a fertilized egg (embryo) is present in the uterus, cells from both the woman and the embryo will secrete specific immune factors to support the embryo’s attachment and penetration into the uterine lining.v

The maternal-fetal interface refers to the point when the uterus and embryonic tissues meet and interact. The immune cells present at this interface promote embryo implantation and prevent rejection of the

How does the immune system affect fertility?

Immune responses in the male and female reproductive tracts are highly regulated by sex hormones. In the uterus, estrogen and progesterone regulate communication between the immune system and uterine tissues to allow embryo implantation and survival of the fetus until birth.vii

Immunological infertility (immune infertility) refers to the inability to conceive or maintain a pregnancy due to immune-related factors. It has become an increasingly recognized issue in infertility. One example involves the female immune system attacking sperm, preventing fertilization. The female reproductive tract can react to semen, leading to an immune response and rejection of the foreign sperm or semen components before fertilization can occur. This activated immune response typically involves immune proteins (anti-sperm antibodies or ASA) that target sperm or components within the seminal fluid.viii

Immune-mediated infertility has also been brought to the forefront of treating women with recurrent implantation failure (RIF) or recurrent pregnancy loss (RPL). Alterations in immune cells have been found in both these populations. Abnormal function of a woman's immune system can also relate to chronic inflammatory diseases such as endometriosis, which occurs in 30-50 percent of infertile women.ix Overall, approximately 20 percent of RPL is linked to irregularities in the maternal immune system.x,xi

It is important to understand that despite some individuals experiencing immunological infertility, the immune system is not an enemy of couples or women trying to conceive. Indeed, an active immune response is important for successful implantation. In addition, when there are immunological imbalances, they are not always due to an over-active or inflammation-promoting immune system.xii

When would a patient see a reproductive immunologist?

Women with unknown causes of infertility, including those who have undergone in vitro fertilization (IVF) and have a history of RIF or RPL, as well as women with known autoimmune disorders who are trying to conceive, are often referred to a reproductive immunologist (RI).xiii

RIs may be able to help these individuals by providing more personalized immunotherapy before subsequent IVF cycles in an attempt to improve reproductive outcomes.

What does a reproductive immunologist do? 

An RI is a medical doctor who specializes in the fields of reproductive immunology and reproductive endocrinology and infertility (REI). These providers focus on immunological clinical care to improve reproductive outcomes.  

An RI can provide immune testing and prescribe immunotherapy treatment, including immunosuppressive drugs like TNFα inhibitors (e.g., Humira®), immunomodulatory therapies such as intravenous immunoglobulins (IVIG) and corticosteroids (e.g., Prednisolone®) to be used during ART treatment.xiv

Immune testing

RIs use a range of immune function tests, from limited panels to more extensive ones. Most immune markers are tested using blood samples, although some RIs also use uterine biopsies.  

A study by Kwak-Kim et al published in the American Journal of Reproductive Immunology described a worldwide survey of IVF clinicians and found that immune tests were recommended to patients in 69 percent of RPL or RIF cases.xv Of these tests, autoimmune testing was the most common.  

Autoimmune tests

Autoimmune tests quantify the blood concentration of certain antibodies that can attack the body’s own cells. These often include the following: antiphospholipid antibodies (anticardiolipin antibodies, lupus anticoagulant, and beta-2 glycoprotein),xvi thyroid peroxidase antibodies (anti-TPO antibodies), and anti-nuclear antibodies (ANA).xvii Not all these antibody tests will be ordered in all cases, it will depend on each patient’s particular diagnosis and history.  For example, anti-phospholipid antibodies may be indicated for some RPL or RIF patients, while anti-TPO antibodies will likely be tested in cases of thyroid imbalances.    


A blood test called an immunophenotype can also be done to measure the quantity of different types of white blood cells in the uterine or peripheral blood environment. Irregularly high quantities could potentially cause infertility or pregnancy loss. For example, some studies indicate that overall B-cell (CD19-positive) levels above 12 percent may lead to infertility or RPL.xviii Another study showed an association between infertility and elevated CD56-positive lymphocytes (immune cells such as NK cells, monocytes, certain T-cells).xix While these associations have been observed, evidence is mixed on potential effective treatments for any abnormalities.

NK cell testing

Natural killer (NK) cells are a type of white blood cell that fight viral infection and tumors. NK cells circulate in the blood stream (called peripheral blood NK cells) but are also found in the uterus during implantation and initial stages of pregnancy (uterine natural killer cells, uNK cells).  

It is possible to analyze the concentration and activity of NK cells. This can be done using a blood test to assess peripheral NK cells, or by performing a uterine biopsy to assess uterine NK cells. Because uterine biopsies are much more invasive and expensive, blood testing is often used to represent uNK cells – however, it is debated whether or not the circulating peripheral NK cells are a good indicator of the uterine NK cells levels/activity.xx

Some researchers believe that an abnormal presence or activity of NK cells may potentially cause implantation or pregnancy failure.xxi Certain studies have shown that elevated levels of NK cells may negatively impact fertility. For example, a study with 137 patients indicated that peripheral NKxxii,xxiii

However, not all evidence supports a link between elevated NK cells fertility. A meta-analysis of 22 studies published in 2013 reported no significant differences in the percentages of peripheral and uterine NK cells in fertile vs. infertile women.xxiv The researchers also reported no significant difference in IVF outcomes between women with and without elevated peripheral NK cells.xxv

Overall, there is lack of consensus as to whether a high percentage of NK cells leads to poor reproductive outcomes.

Th1:Th2 assay

The Th1:Th2 assay is a test used to determine the ratio between two types of T helper cells (Th1 & Th2) to identify an imbalance in the maternal immune system.  

In general, Th1 creates a pro-inflammatory environment whereas Th2 creates an anti-inflammatory environment.xxvi  There is some evidence to suggest that higher Th1:Th2 ratios may be associated with subfertility and increased risk of miscarriage.xxvii

Endometrial Immune Profiling  

A new technology called endometrial immune profiling analyzes the local immune response in the endometrium during the window of implantation using endometrial tissue biopsy. It aims to identify patients with an abnormal uterine immune environment (both over- and under- reactive) in order to help design personalized treatment plans. In a study by Ledee et al (2020) of 1,738 infertile patients receiving endometrial immune profiling, 28 percent of women had an under-active immune environment and 45 percent had one that was over-active. Patients with a history of RIF and an irregular endometrial immune profile had improved pregnancy rates (38 percent vs. 27 percent) after receiving personalized immunomodulatory treatment.xxviii


For women with identified immune imbalances, as well as for those with unexplained RIF or RPL, immunomodulating and immunosuppressive therapies can be used to help improve fertility outcomes. These therapies include immunomodulatory intravenous immunoglobulins (IVIg), intravenous intralipids, and immunosuppressive drugs such as Prednisone, Prednisolone®, Humira® or Prograf®.xxix,xxx


Intravenous immunoglobulin (IVIg) therapy is a product containing a mix of antibodies (immunoglobulin G, IgG) derived from thousands of donors. It is administered directly into a vein (intravenously) and used to treat immune deficiencies (e.g., due to cancer treatment), as well as autoimmune and inflammatory disorders. IVIg is sometimes recommended for IVF patients with RPL or RIF, especially if associated with immune causes, since IVIg has both anti-inflammatory and pro-inflammatory effects on the immune system.  

Some studies have shown altered cellular profiles following IVIg treatment,xxxi and a potential for improved outcomes in RPL patients with immune abnormalities.xxxii However no large IVIg randomized clinical trials have shown an improvement in live birth or miscarriage rates in patients with unexplained RPL.xxxiii,xxxiv

A universal protocol for IVIG treatment does not exist, but most studies using IVIg administer it in 1–4-week intervals with a dosage of 400 mg/kg. A common protocol for patients with RIF and RPL is at least one IVIg infusion before ovulation or embryo transfer, one infusion after a positive pregnancy test, and then an infusion every 1-3 weeks during pregnancy. Clinics can adjust the protocol and dosage for IVIg depending on immune testing results such as NK cell levels, Th1/Th2 ratios, or autoimmune status.xxxv


Corticosteroids, such as prednisone or prednisolone, suppress the immune system and prevent inflammation. A large systematic review found no benefit of adding corticosteroids to IVF cycles for the indication of improving success rates.xxxvi  

Corticosteroids may be prescribed to RIF or RPL patients with increased NK cell activity. A study published in the Journal of Reproductive Immunology (2018) reported that only 55 percent of RIF patients responded positively to prednisone use (as indicated by reduced uNK cell activity). The researchers concluded that only a portion of RIF patients may benefit from corticosteroid treatment.xxxvii Additionally, no study has evaluated how prednisone therapy affects pregnancy rates in RIF patients, although a randomized trial is ongoing.xxxviii For patients with unexplained RPL, prednisone may be beneficial when given in combination with other treatments. A double-blind placebo randomized control trial including 160 patients with RPL reported that patients who received prednisone, low dose aspirin, and heparin had a higher ongoing pregnancy rate (70 percent) compared to those who received heparin and aspirin alone (9 percent).xxxix Importantly, this was a small study. There are no larger trials to date evaluating the use of prednisone in unexplained RPL. One trial was proposed, but results have not yet been published.xl

The definition of immunotherapy

Corticosteroids may be useful in women with RPL and known cellular immune abnormalities. A study published in the Journal of Reproductive Immunology (2021) evaluated whether immunomodulatory treatment (prednisone alone, 10 mg daily orally, or combined with 400 mg/kg of IVIG) and anticoagulation (40 mg daily of Lovenox® and 81 mg daily of aspirin) treatment improved IVF outcomes for 197 RPL or RIF women with known cellular immune abnormalities and thrombophilia (a blood clotting disorder commonly associated with autoimmune diseases). They observed that for all patients who received immunomodulatory treatment, there was a significant improvement in pregnancy rate (48 percent vs 33 percent in controls) and live birth rate per fresh or frozen embryo transfer cycle (40 percent vs 2 percent in controls). They concluded that immunomodulatory and anticoagulation treatment significantly improved reproductive outcomes of IVF cycles for women with known immune causes of RPL and/or RIF and thrombophilia.xli

Prednisone may be useful in the setting of male factor infertility. Studies have shown improved sperm parameters and pregnancy rates for men with anti-sperm antibodies after treatment with prednisone.xlii Corticosteroids can also improve sperm counts in men with low sperm concentration (oligospermia) and signs of local inflammation on ultrasound.xliii

Intralipid therapy

Intralipid therapy involves the IV infusion of a sterile fat emulsion (20 percent fat emulsion diluted in 250 mL of saline), which is suggested by several studies to suppress NK cell count and activity in patients with RIF and RPL. A randomized controlled trial by Singh et al (2019) evaluated 105 women with a history of failed implantation in previous IVF cycles. They found higher clinical pregnancy rates in those who received two intralipid doses compared to those who did not (34 percent versus 14 percent).xliv In contrast, a 2019 clinical trial and 2020 meta-analysis did not observe any significant difference in pregnancy outcomes for those using intralipids compared to controls.xlv,xlvi

Overall, there is a paucity of research on intralipid treatment, and the limited studies have conflicting results.xlvii

How to find a reproductive immunologist?

Finding an RI can be difficult in certain countries. Some fertility clinics conduct immune testing themselves or are associated with specific reproductive immunology programs with RIs. If this is the case, a woman's fertility doctor may refer her to the RI if indicated or if requested by the patient. Other women undergoing assisted reproduction may choose an RI based on personal recommendations or online searches.  

Individuals and couples interested in reproductive immunology should check with their IVF clinics for a referral.


Understanding the interplay between the reproductive and immune systems is generally quite complex, yet for those hoping to achieve a viable pregnancy, it can be critical to know the basics. Doing so will enable women or couples undergoing IVF to consult with an RI if they suspect an immune system disfunction related to infertility, and to know what questions they should be asking, as well as what testing, and treatments are available.

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November 23, 2023