Some of the cells that pass between a mother and child during pregnancy, and in some cases between siblings through the mother’s body, do not disappear. They can remain in the body for years, even decades.

Researchers have found that some of these cells do not stay scattered or inactive. In certain cases, they appear in higher numbers in damaged or diseased tissue, including the liver, heart, skin, and other organs.

That pattern has led scientists to look more closely at what they are doing there.

In some studies, these cells have been shown to take on characteristics of the surrounding tissue, expressing markers similar to local cells. In others, they appear to behave in ways that resemble stem or progenitor cells, responding to chemical signals released by injury.

In simple terms, when tissue is damaged, it sends out signals. Some of these long-lived cells appear to respond to those signals, move toward the site, and begin to integrate into the area.

What is not yet clear is what role they ultimately play.

Some researchers think they may contribute to repair or regeneration. Others suggest they may be involved in immune responses or inflammation. It is also possible that, in some contexts, they do very little at all.

The evidence is still developing.

What is clear is that these cells are not always passive. They can respond, move, and in some cases begin to resemble the tissue around them.