The immune cells, known as Tregs, were thought to control inflammation.
But, now researchers believe they also triggers stem cells in the skin to promote healthy hair growth.
Without the immune cells, the scientists found stem cells cannot regenerate hair follicles, leading to baldness.
Dr Michael Rosenblum, who led the research at the 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.”