Bacteria inflaming the meninges have developed an immunosuppressive mechanism that contributes to their ability to attack the brain. Researchers found that, by activating pain receptors (nociceptors) to release chemical substances that block an immune cell receptor, Streptococcus pneumoniae and Streptococcus agalactiae deactivated the protective function of macrophages and weakened brain defenses. This, in turn, enabled them to invade the brain.