Reproductive Immunology: How Does the Immune System Affect Pregnancy?

Reproductive immunology is a rapidly evolving subspecialty that studies the relationship between the immune system, fertility, pregnancy, and pregnancy loss. It has become central to understanding and managing unexplained infertility, recurrent IVF failure, and recurrent miscarriage — situations in which the immune system plays a delicate role in either accepting or rejecting the embryo.

What is reproductive immunology?

In a healthy pregnancy, the embryo carries half of its genes from the father — making it partly "foreign" from the perspective of the maternal immune system. For pregnancy to succeed, the maternal immune system must adapt to accept the embryo without attacking it, through complex mechanisms including regulatory T cell modulation, balance of uterine natural killer (uNK) cells, and dampening of the inflammatory response.

When this adaptation fails, problems can range from difficulty with implantation, early pregnancy loss, pre-eclampsia, and intrauterine growth restriction. ESHRE guidelines on recurrent pregnancy loss highlight immune factors as worth investigating, particularly in recurrent cases.

How the immune system affects fertility

Autoantibodies

The body can produce antibodies targeting elements important for fertility:

• Antiphospholipid syndrome (APS) — the most clearly established immunological cause of recurrent miscarriage
• Anti-thyroid antibodies (Anti-TPO, Anti-TG) — associated with higher miscarriage rates even when thyroid function appears normal
• Anti-ovarian or anti-sperm antibodies

Uterine natural killer (uNK) cells

These cells play a normal physiological role in placental blood vessel formation. Abnormal activity may contribute to some implantation failures, but the evidence is less definitive and still under active investigation, as noted by ASRM.

Excessive inflammatory response

Elevated inflammatory mediators (such as TNF-α and IL-6) in the endometrium can create an environment unsuitable for embryo implantation or pregnancy maintenance. This requires specialist evaluation.

Endometriosis as an immune-mediated condition

Endometriosis is increasingly understood as a chronic immune-mediated condition that contributes to subfertility through chronic inflammation and effects on egg quality and the tubal environment.

When immunological evaluation is warranted

Not every case of infertility needs an immune workup. According to ESHRE and RCOG, evaluation is recommended in:

• Recurrent miscarriage (two or more consecutive losses)
• Two or three failed IVF cycles with good-quality embryos
• Recurrent implantation failure (RIF) with euploid blastocysts
• Previous severe or early-onset pre-eclampsia
• Previous intrauterine growth restriction
• A diagnosed autoimmune disease (lupus, Hashimoto's thyroiditis, rheumatoid arthritis)

Diagnosis

Evaluation should be focused and evidence-based, avoiding unnecessary testing:

• Antiphospholipid syndrome (APS): anti-cardiolipin, anti-beta 2 glycoprotein I, lupus anticoagulant — repeated at 12 weeks to confirm
• Thyroid function and antibodies: TSH, free T4, anti-TPO, anti-TG
• Extended coagulation studies: protein C and S, antithrombin III, Factor V Leiden, MTHFR variant, prothrombin G20210A mutation
• Immune-related vitamins: vitamin D, vitamin B12
• Peripheral or endometrial NK cell analysis — only in specialist centres, with careful interpretation
• Inflammatory markers: CRP, homocysteine

Modern immunotherapy

Treatments are selected precisely based on test results and are not general recommendations for every patient.

Evidence-based therapy

• Low-dose aspirin and low molecular weight heparin (LMWH): the established treatment for APS, with documented improvement in live birth rates
• Thyroid optimisation, including treatment of TPO antibodies when indicated
• Correction of vitamin D deficiency

Therapy for selected cases

• Intralipid infusions: a lipid emulsion used in some cases to modulate NK cell activity, with mixed evidence
• Intravenous immunoglobulin (IVIG): reserved for specific severe cases with informed consent
• Low-dose corticosteroids: used in selected protocols

Endometriosis-directed treatment

Laparoscopic excision of endometriotic lesions, or use of GnRH agonists before IVF cycles to improve outcomes in advanced disease.

Safety of immune therapy

RCOG and ESHRE guidelines stress that immunotherapy must be based on clear evidence and a specific diagnosis. Empirical use in patients without a confirmed indication is not only ineffective but may carry risks. Evaluation in specialised reproductive immunology centres is therefore preferred.

Lifestyle support

Lifestyle directly affects immune function:

• Weight management to reduce chronic inflammation
• A diet rich in antioxidants (leafy greens, berries, nuts)
• Adequate omega-3 intake
• 7 to 8 hours of sleep
• Stress management through mindfulness and exercise
• Avoiding smoking and alcohol

When to consult a specialist

Reach out to a fertility specialist for immune evaluation if you have:

• Two or more consecutive miscarriages
• Two or more failed IVF cycles with good-quality embryos
• A personal or family history of autoimmune disease
• Previous pre-eclampsia
• Previous intrauterine growth restriction

Frequently asked questions

Does every infertility patient need immunology testing?

No. Immunology testing is not a first-line screen. It is performed when clear clinical indicators exist, to avoid misdiagnosis and unnecessary treatment.

Does immunotherapy guarantee pregnancy?

No fertility treatment guarantees a pregnancy. But when APS is accurately diagnosed, aspirin and heparin can increase the live birth rate from about 25% to more than 70%, according to published trials.

Is Intralipid proven?

Evidence is mixed. ESHRE does not recommend routine use; it should only be considered within specialist centres in selected cases with informed consent.

Can the immune system reject a baby because of blood type differences?

Rh incompatibility is a well-known scenario managed with prophylactic anti-D injections. The notion that ABO differences cause infertility is not supported by strong evidence.

Sources

• ESHRE Guideline on the management of recurrent pregnancy loss.
• National Institute for Health and Care Excellence (NICE). Fertility problems guidance.
• Royal College of Obstetricians and Gynaecologists (RCOG). Green-top Guideline No. 17: The investigation and treatment of couples with recurrent first-trimester and second-trimester miscarriage.
• American Society for Reproductive Medicine (ASRM). Practice Committee statements on NK cells and recurrent implantation failure.
• Updated Sapporo criteria for antiphospholipid syndrome diagnosis.
• Cochrane Database of Systematic Reviews. Immunotherapy for recurrent miscarriage.

Medical disclaimer

This article provides general medical information for educational purposes only. It is not a substitute for personal medical advice, diagnosis, or treatment from a qualified healthcare professional. Immunotherapy decisions are complex and should be made in partnership with a specialist. Please consult Dr. Haytham Ibrahim before making any decisions about immune evaluation or treatment for fertility.