In 1702, Queen Anne of England ascended to the throne. Over the course of her life she experienced 18 pregnancies. Not a single living heir survived. Five children were stillborn, eight died in infancy, and the rest ended in miscarriage. Modern medical historians believe the cause may have been antiphospholipid syndrome — an autoimmune condition in which the immune system attacks its own cells. Today, this is diagnosed with a blood test and is treatable.
To protect us from thousands of unknown pathogens — viruses, bacteria, parasites that did not yet exist when our immune system evolved — evolution took a radical approach. It made immune cells random.
Every T- and B-lymphocyte carries a unique receptor assembled from genetic fragments by random recombination — as if you shuffled the letters of the alphabet and each time produced a new word. The result: trillions of unique cells theoretically capable of recognising any enemy, including those humanity has never encountered before.
But randomness has an unavoidable cost. Some receptors, formed by chance, end up directed against the body’s own tissues. A lymphocyte that should have hunted a virus instead targets pancreatic cells, nerve fibres, or blood vessel walls.
The immune system builds its army at random — and then spends a lifetime making sure that army does not turn against its own.
The body manages this in several ways. The primary one is the thymus: an organ where young immune cells undergo something like a security check. Those that react too aggressively to the body’s own tissues are destroyed. But the system is imperfect — some self-targeting cells pass the screening and enter the bloodstream. This is why autoimmune diseases exist, and why a second layer of control was needed.
In 1995, Japanese immunologist Shimon Sakaguchi published results that were initially met with scepticism. He had found that among T-lymphocytes there exists a distinct subpopulation — small, around 5–10% of the total, but with disproportionately large influence. He called them regulatory T-cells, or T-regs.
The proof of their role was elegant. When T-regs were removed from healthy mice, the animals developed systemic autoimmune disease: the body began attacking multiple organs simultaneously — the thyroid, the gut, the adrenal glands. When T-regs were restored, the attack stopped. This was the first direct experiment demonstrating that the immune system contains a built-in mechanism of self-restraint.
The key to these cells is the gene FOXP3. It acts as the master switch: without it, T-regs neither form nor function. Mice with a FOXP3 mutation develop a lethal autoimmune assault on all organs within weeks of birth. In humans, the equivalent mutation causes IPEX syndrome — a severe multi-organ autoimmune condition that is fatal in infancy without treatment.
What exactly do T-regs do? They secrete anti-inflammatory molecules — interleukin-10 and TGF-β — and directly suppress the activity of other immune cells. If the immune system is an orchestra, T-regs are the conductors keeping the brass section from drowning out everything else.
From the perspective of classical immunology, pregnancy is an immunological scandal. The foetus carries the father’s genes — meaning it is genetically half-foreign to the mother. By every rule of immune response, it should be rejected the way a transplanted organ from an incompatible donor is rejected. Yet it is not. Why?
The answer is multi-layered, and T-regs are one of its key components. In early pregnancy, the concentration of regulatory T-cells rises sharply in the decidua — the specialised uterine tissue formed for implantation. They create a local zone of immunological tolerance: the maternal immune system continues to fight pathogens, but does not attack the embryo.
Pregnancy is not a weakening of immunity. It is its highest achievement: learning to protect both the mother and a genetically foreign foetus simultaneously.
The placenta’s own recognition mechanism is equally remarkable. Trophoblast cells — the tissue from which the placenta forms — express a unique molecule called HLA-G, found nowhere else in the adult body. HLA-G acts as a kind of pass: it signals maternal NK-cells not to attack. This mechanism was described in 1990 by Peter Loke and Ashley Moffett and has since become one of the central subjects of reproductive immunology.
Understanding the normal allows us to understand the pathological. Several reproductive complications are now viewed through an immunological lens.
In women who have experienced two or more consecutive miscarriages, researchers consistently find reduced levels of T-regs in the endometrium compared with women who carry pregnancies to term. The causal link has been confirmed: in mice with artificially reduced T-reg numbers, miscarriage rates rise substantially, and restoring the T-reg population normalises pregnancy outcomes.
This serious complication of late pregnancy — sudden hypertension, kidney damage, and risk to the lives of both mother and child — also has an immunological component. Women with pre-eclampsia show a deficit of T-regs and elevated activity of pro-inflammatory cells. The placenta in this condition forms abnormally — and research indicates the immune imbalance precedes the clinical symptoms rather than follows them.
One of the most discussed scenarios in reproductive medicine: a genetically normal embryo, normal endometrial morphology — and yet the transfer does not result in pregnancy. A portion of such cases involves immunological disturbances at the level of the endometrium: excessive NK-cell activity, T-reg deficiency, autoimmune antibodies.
The immune system is not just a defence against infection. It is one of the central participants in pregnancy: it decides whether the uterus accepts the embryo, whether the placenta establishes normal blood flow, whether pre-eclampsia develops. Understanding these mechanisms changes how we approach both pregnancy planning and the investigation of repeated failures.
Most immunological disorders affecting reproduction are detected by standard tests and corrected with appropriate therapy. The question is whether the right questions are being asked of the right specialist.
Verified reproductive immunologists working to evidence-based standards are available in the Partners section. Module 6 (Hormonal Navigator) includes an expanded immunological screening section — including for recurrent miscarriage and repeated IVF failure.
a subpopulation of T-lymphocytes that suppress excessive immune responses. A key mediator of immunological tolerance during pregnancy.
the master switch for T-reg formation and function. Mutations cause the severe IPEX syndrome.
a cell-surface molecule unique to trophoblast cells. Signals maternal NK-cells not to attack the placenta.
autoimmune condition raising placental thrombosis risk. The leading diagnosable immunological cause of recurrent miscarriage.
specialised NK-cells in the endometrium. In normal amounts they support implantation; in excess they may impede it.