Abstract
This chapter summarizes the role of immune cells of the central nervous system (CNS) in maintaining immune response. An effective innate immune response is crucial for the elimination of pathogens and is vital for survival in the CNS. The CNS is considered to be an immune-privileged organ system, for its absence of lymphatic irrigation, the low expression of major histocompatibility complex molecules, and the presence of a blood-brain barrier. Microglia, the resident immune cells of the CNS, are of mesodermal origin and constitutes about 10% of total glial cell population. Microglia constitutively expresses low levels of immune receptors essential to the initiation and propagation of immune responses. Toll like receptors, which are essential for mounting an immune response against infection, control microglial activation and also regulate microglial death following pathological activation. The harmful effect of pathogen invasion is counteracted by the host inflammatory reaction, which is inevitably associated with tissue injury.