Researchers from the University of Cambridge in the UK and the US National Institute of Mental Health have discovered that chronic stress causes immune cells known as neutrophils to be released from the skull bone marrow.

These cells later accumulate in the meninges and contribute to the symptoms of depression and anxiety.

In an experiment on mice exposed to social stress, scientists observed a significant increase in the levels of these cells, which persisted even after the stress period ended. They explained that the neutrophils in the brain differ from those circulating in the bloodstream, as they directly affect the animals’ mood.

This mechanism is linked to the activation of the type I interferon signaling pathway within the neutrophils, which acts as an “alarm” for the immune system. When researchers blocked this pathway, depression-like symptoms decreased. Doctors note that this immune “pulse” is similar to the known side effects in hepatitis C patients receiving interferon treatment, many of whom suffer from depression.

Stacey Keigar from the University of Cambridge’s School of Medicine confirmed that the research results reveal the importance of the interaction between the immune and nervous systems and open the door to developing new treatments for mood disorders by targeting immune cells in the bone marrow, not just brain chemistry. She added, “It is known that antidepressants do not help nearly one-third of patients, but by understanding what happens to the immune system, we may find more effective ways to alleviate depression symptoms.”