For some mice, life produces anxiety—where to nest, which bag of chips to chew through, how to escape human detection. To cope with such crippling panic, these mice groom themselves so thoroughly and frequently that they lick off their fur. These rodents develop bald patches and sores over their bodies. Even in pain, the mouse licks and licks.

Researchers, led by 2007 Nobel prize winner for medicine, Mario Capecchi, found that with a bone marrow transplant, the mice stop their obsessive-compulsive behaviors and live normal lives. Capecchi says this shows a strong connection between the immune system and mental illness.

Capecchi bred the mice with a mutation Hoxb8, giving them the equivalent of human OCD. Hoxb8 originates in microglia, which are immune cells. Most would assume that neurologic cells would control behavioral ticks and the group was surprised to learn otherwise. The mutant mice also had 15 percent fewer Hoxb8 than regular mice. To examine Hoxb8's effect, the researchers injected bone marrow from mice with mutant Hoxb8 mice into healthy mice. The healthy mice began licking themselves compulsively, pulling out their fur and creating sores. In turn, the researchers injected the mutant mice with healthy bone marrow, and these mice became non-obsessive.

Even though bone marrow transplants possess too many risks to make it a viable treatment for OCD, Capecchi says this understanding might help researchers develop drugs that regulate microglia—perhaps providing better treatments for OCD.