Microglia: mechanisms of activation and significance in pathogenesis of neuropsychiatric illnesses

Microglia are the resident macrophages of the brain. They are the central cellular element to initiate defense mechanisms against destructive environmental influences and to facilitate regenerative processes. No other cell type of the brain is endowed with a comparably comprehensive, immunocompetent machinery like microglia. It encompasses cell proliferation, migration and differentiation into full-blown macrophages able to present antigen and to phagocytose cell debris. Relatively little is known about these stages of microglia activation on the cellular and molecular level, although microglia have been described as a separate cell type of the brain as early as in the 30ies of this century by P.del Rio Hortega. This review summarizes the data that have accumulated until now in this respect and tries to embed them into a clinical framework. Special focus has been given to the role of this cell type in the development and progression of Multiple Sclerosis, HIV-associated dementia and Alzheimer's disease.