Picture a crime scene at which a police officer is the criminal, indiscriminately killing bystanders who can't flee. Then, the rash officer calls in reinforcements, who not only shoot at passersby but poison some of them.
The berserk police are errant immune cells and their innocent victims are sheaths of a substance called myelin that surrounds axons—the impulse-carrying tendrils extending from neurons, or nerve cells. Damaging myelin sheaths kills axons, resulting in the numbness, weakness, slurred speech, and paralysis of multiple sclerosis.
Two studies now indicate that a second wave of immune-cell carnage follows this initial mutiny. In this attack, immune cells produce copious amounts of glutamate, a transmitter of neural signals. The overabundance wreaks havoc on nerve tissue, overpowering the resident nervous-system cells that make myelin.