Neuroimmunology is the study of the interaction between the immune system and the nervous system. Entry and function of immune mediators into the central nervous system are dependent on specific molecule-mediated communication between the two systems occurring at the level of the blood–brain barrier and within the brain parenchyma. Immune surveillance does occur within the central nervous system, protecting the host from infectious agents. An activated immune response within the central nervous system can have the dual roles of effecting tissue injury or contributing to neural protection and recovery from injury.