The researchers were investigating the role played by anti-inflammatory immune cells called Tregs in skin health generally.

They found a way to temporarily remove the Tregs from the skin of laboratory mice, who had been shaved to allow the effects to be observed.

But the scientists then noticed something unexpected – the hair failed to grow back.

Previously it was thought that stem cells cause hairs to regrow after they fall out, but the team discovered that this only happens if Tregs are present.

One of the scientists, Professor Michael Rosenblum, an immunologist and dermatologist at University of California San Francisco, said: “Our hair follicles are constantly recycling. When a hair falls out, the whole hair follicle has to grow back.

“This has been thought to be an entirely stem cell-dependent process, but it turns out Tregs are essential.

“If you knock out this one immune cell type, hair just doesn't grow.

“It’s as if the skin stem cells and Tregs have co-evolved, so that the Tregs not only guard the stem cells against inflammation but also take part in their regenerative work.

“The stem cells rely on the Tregs completely to know when it's time to start regenerating.”

The researcher believe that defects in Tregs could be responsible for the immune disease, alopecia areata, which causes hair to fall out in patches and possibly also play a part in other kinds of baldness.

The same stem cells that regrow hair are also involved in healing damage to the skin, so Tregs may also be involved in this process.

Tregs’ role – as previously understood – was mainly to regulate the immune system, helping it tell what to attack and what to leave alone.

When they malfunction it can lead to allergies to peanuts and other harmless substances or cause the immune system to attack the body.

Professor Rosenblum and colleagues had previously showed that Tregs help the immune systems of baby mice learn which skin microbes are not harmful and also that they secrete molecules that help heal wounds.

They were investigating these effects further when they noticed that patches of shaved hair on the lab mice were not regrowing.

“We thought, ‘Hmm, now that’s interesting,’” Professor Rosenblum said. “We realised we had to delve into this further.”

Using sophisticated imaging techniques, the researchers were able to show that Tregs gathered around follicle stem cells at the start of the process to regrow a hair.

When Tregs were removed from the skin, this prevented the regrowth of hair – but only if this was done within three days of the hair being shaved. After this time, the hair would regrow normally despite the absence of Tregs.

The cause of alopecia is poorly understood, but previous studies have showed genes associated with the condition are mostly related to Tregs. Boosting Treg function has been found to help.

Professor Rosenblum suggested further research into Tregs’ role could lead to improved treatments for hair loss generally and better understanding of their role in wound healing.

“We think of immune cells as coming into a tissue to fight infection, while stem cells are there to regenerate the tissue after it's damaged,” he said.

“But what we found here is that stem cells and immune cells have to work together to make regeneration possible.”

The research was described in the journal Cell.

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