Scientists have discovered a novel mechanism and role in the brain for hyaluronic acid -- a clear, gooey substance popularized by cosmetic and skin care products. Hyaluronic acid may be the key in how an immune signal moves from the blood stream to the brain, activating the brain's resident immune cells, the microglia. Findings from this study have important implications for better treatments for stroke, neurodegenerative diseases, as well as head injuries.