Microglia: key innate immune cells of the brain.

Microglia are the histiocytes of the central nervous system. These long-lived cells undergo very little turnover in normal physiological states; however, in pathological conditions, increased egress from the bone marrow and chemoattractive signals in the brain can substantially modulate the indigenous population. Although they were initially conceived of as "resting" cells, recent data suggest that they would be more aptly described as "surveillance" cells. Microglia are specifically adapted to sense various types of danger and differentially react with a classical or alternative reparative response. Our understanding of macrophage function has shifted away from focusing on cell lineage to a more systems-based biology of gene networks accomplishing the detoxification and immune functions. With our greater appreciation of microglial involvement in the innate immune response, we have entered a new era in which the modulation of microglia can be proposed as a means of modulating neurological disease.