Pain is a way for our bodies to tell us that we are hurt, and we should take care to protect ourselves from further harm. This signal manifests in the form of inflammation and hypersensitivity to touch, at the site of injury. Once the injury is healed, the associated symptoms also resolve themselves. But in some cases, even after the inflammation subsides, the pain persists for months or years, which can lead to decreased quality of life for people with this long-term (or chronic) pain. While physical therapy and medication can help relieve pain, they are unreliable at best and can sometimes lead to opiod addiction.

To find robust methods of pain relief, we need to understand how our bodies resolve pain naturally and how that process is disrupted in case of chronic pain. In a recent study (currently posted as a pre-print) by scientists based in the Netherlands and the US, researchers unraveled how immune cells, called macrophages, relieve pain caused by damage to sensory cells. 

Macrophages are known for their ability to ‘eat’ foreign particles in our body. They also play a substantial role in both the initiation and mitigation of pain. Two types of macrophages are involved in these processes: M1 and M2. The former initially crowd around a wounded area, causing inflammation and pain. As the wound heals they are then replaced by M2 macrophages, which ease these symptoms. The researchers found that an imbalance in the levels of these two types of macrophages can cause chronic pain. 

The way that M2 macrophages alleviate pain is fascinating. A wound disrupts the energy-producing cellular machinery, called mitochondria, of surrounding sensory cells. M2 macrophages actually transfer their own mitochondria into the affected cells in small balloon-like structures, easing the pain sensation. This suggests that therapies aimed at increasing the mitochondrial transfer from M2 macrophages into cells around our wounds, giving them a jolt of energy, could help relieve chronic pain.