Why do your mother’s cells stay in your body for life?

Uma pregnancy ends, the umbilical cord is cut, but the physical bond between mother and child continues - active, invisible and silent.

Inside many adults alive today, cells from their mother still circulate, passed on during pregnancy and tolerated by the body decades after birth. Science is beginning to understand how this unlikely co-existence is maintained and what it might reveal about immunity, disease, and even our own biological identity.

What is maternal microchimerism?

Researchers call this phenomenon maternal microchimerism. Put simply, it is the presence of a small number of a mother’s cells in her child’s body long after pregnancy has ended.

These cells cross the placenta during pregnancy and settle in different tissues of the foetus. After birth, they do not disappear completely. Studies show they can still be found decades later.

On average, it is estimated that around one in every million cells in a given area of the body may be of maternal origin - an almost invisible minority, but biologically active.

The phenomenon works both ways. The mother also retains cells from the foetus, which can remain in her body for many years. This two-way traffic creates a kind of shared “biological archive” across generations.

Where these maternal cells hide

Maternal cells have already been identified in different parts of the child’s body, including:

• Liver
  
• Heart
  
• Skin
  
• Peripheral blood
  
• Brain and central nervous system
  

These are not large colonies of maternal tissue, but small, scattered cellular “islands”. Even so, studies suggest the impact may be disproportionate to their small numbers.

Why the immune system does not destroy them

In theory, the immune system should see these cells as invaders. They carry the mother’s genes, which differ from the child’s “standard” genetic profile. Immunology’s basic logic says the body should attack anything that looks foreign.

In practice, something different happens. Most of these cells are tolerated, sometimes for a lifetime. Research in mice, published in the journal Immunity, helps explain this apparent contradiction.

The role of “educator” cells

In the study, scientists followed the behaviour of maternal cells originating from bone marrow. They display specific markers, such as LysM and CD11c, typical of myeloid or dendritic-like cells specialised in coordinating immune responses.

During a very early window of life, these cells act like “teachers” for the developing immune system. In contact with the foetus’s body, they promote the development of a specific group of lymphocytes called regulatory T cells, or Tregs.

Regulatory T cells act as the brakes of immunity: they signal to the body that certain foreign presences should not be attacked, helping to prevent excessive reactions.

With this fine-tuning, the baby’s immune system learns that those maternal cells do not represent a real threat. Tolerance takes hold and, under normal conditions, is maintained throughout life.

When researchers removed, in the laboratory, the specific maternal LysM⁺ CD11c⁺ cells, the balance collapsed. The number of Tregs plummeted and, suddenly, the animals’ bodies began rejecting the mother’s cells, treating them as invaders.

An active tolerance, not an automatic one

These findings suggest that tolerance of maternal cells is not merely a side effect of pregnancy. It is an active process, dependent on a small group of highly specialised cells.

If these “educators” disappear or lose function, the immune system may turn against what was previously accepted. This helps link maternal microchimerism to certain inflammatory and autoimmune conditions.

Between protection and risk: the ambiguous side of maternal cells

The role of these cells seems to have two sides. In some contexts, they contribute to tissue repair. In others, they are associated with autoimmune diseases such as lupus, thyroiditis and systemic sclerosis.

Researchers have detected maternal cells in areas affected by chronic inflammation, neurological disorders and even in some types of cancer. The key question is whether they are:

• worsening the condition by triggering confused immune responses, or
• trying to contain the damage by supporting healing and controlling inflammation.

A related set of phenomena has also been studied in the context of transplants. The idea is that understanding how the body learns to tolerate the mother’s cells could help refine medicines and protocols that prevent rejection of donated organs and tissues.

A blurred boundary between “self” and “other”

Microchimerism forces us to rethink biological identity. Genetically, an individual is not made only of their own cells. They carry a mosaic: material of maternal origin, sometimes from previous pregnancies, and even from lost twins at very early stages.

This mix of genetic codes creates a grey zone: foreign cells that become, in practice, a stable part of the body.

Culturally, the image of “little pieces of mum” scattered through a child’s body reinforces a physical dimension of family bonds that does not end when the umbilical cord is cut.

What this research could change in practice

By mapping the behaviour of maternal cells more precisely, scientists see opportunities across several areas of medicine:

Area	Possible applications
Autoimmune diseases	Determine whether maternal cells are involved in triggering disease or protecting against flare-ups.
Transplants	Inspire new ways to induce tolerance to donor organs and tissues, reducing rejection.
Oncology	Investigate whether maternal cells in tumours act as allies of the immune system or accomplices to cancer.
High-risk pregnancy	Better understand inflammation-linked complications, such as pre-eclampsia, through mother–foetus cellular flows.

In the long term, this knowledge could support therapies that do not simply “switch off” immunity, but modulate it in a way similar to what nature does during pregnancy: training targeted tolerance without leaving the body vulnerable to opportunistic infections.

Concepts worth an extra explanation

For readers outside biomedicine, a few terms appear frequently in this debate and are worth unpacking:

• Microchimerism: the presence of a small number of cells that are genetically different from the rest of the body, yet remain stably integrated.
  
• Regulatory T cells (Tregs): a subtype of T lymphocytes that reduce excessive defence reactions, helping prevent the body from attacking itself.
  
• Dendritic cells: immune cells that act as messengers, presenting “clues” about invaders to other defence cells.
  
• Immune tolerance: a state in which the defence system learns not to react against certain targets, even if they appear foreign.
  

It can help to picture this as a school: the young immune system arrives “raw” and, during pregnancy and childhood, goes through intensive lessons. Maternal cells are part of the teaching staff, helping instruct what should be attacked and what should be spared.

Future scenarios and open questions

One line of research asks whether it might one day be possible to use mother-derived cells as a programmed therapeutic tool. In theory, they could carry specific instructions to dampen inflammation in strategic places such as joints or the gut.

Another area examines less obvious risks. In families with a strong history of autoimmunity, understanding a person’s microchimerism profile might help identify who is more likely to develop certain diseases. That raises ethical questions: to what extent is it desirable to map and anticipate this kind of predisposition?

While answers are still emerging, one fact already stands: pregnancy leaves more than memories and family stories. It also leaves a cellular legacy mixed into the body - at levels most people will never notice, but which may influence health throughout a lifetime.