Microglia are a type of glial cell that acts as the first and main form of active immune defense in the central nervous system (CNS).
Microglia constitute 20% of the total glial cell population within the brain. Unlike astrocytes, individual microglia are distributed in large non-overlapping regions throughout the brain and spinal cord.[1] Microglia are constantly moving and analyzing the CNS for damaged neurons, plaques, and infectious agents.[2] The brain and spinal cord are considered “immune privileged” organs in that they are separated from the rest of the body by a series of endothelial cells known as the blood-brain barrier, which prevents most infections from reaching the vulnerable nervous tissue. In the case where infectious agents are directly introduced to the brain or cross the blood-brain barrier, microglial cells must react quickly to increase inflammation and destroy the infectious agents before they damage the sensitive neural tissue. Due to the unavailability of antibodies from the rest of the body
(antibodies are too large to cross the blood-brain barrier),

microglia must be able to recognize foreign bodies, swallow them, and act as antigen-presenting cells activating T-cells. Since this process must be done quickly to prevent potentially fatal damage, microglia are extremely sensitive to even small pathological changes in the CNS.[3] They achieve this sensitivity in part by having unique potassium channels that respond to even small changes in extracellular potassium.[2]

http://www.answers.com/topic/microgl...80.99s_disease

Parkinson’s disease
Parkinson’s disease is a movement disorder in which the dopamine producing neurons in the brain, don’t work properly.[18] The area of the brain affected by Parkinson’s is called the substantia nigra. It is here that the neurons either become impaired or die.[18] The substantia nigra has one of the highest concentrations of microglia in the brain.[14]

Activated microglial cells have been found around extraneuronal neuromelanin released from impaired dopaminergic neurons in the substantia nigra of patients with Parkinson’s disease.[19] A study by Henrik Wilms discovered that neuromelanin acts as a chemoattractant for microglial cells and induces morphological transformation of microglia cells to an activated state.[19] Neuromelanin also induces synthesis of proinflammatory microglial molecules.[19] All of the inflammatory compounds that are up-regulated in Parkinson’s disease can be produced by microglia, especially activated microglia.[14]

Another study conducted by Wei Zhang stated, “…We have shown for the first time aggregated α-synuclein, the major components of Lewy bodies in patients with Parkinson's disease or dementia with Lewy bodies, activated microglia leading to enhanced dopaminergic neurotoxicity.”[20]

External links
Microglia home page at
microglia.net
http://www.microglia.net/
The Role of Microglia in the Central Nervous System - Clinical Microbiology Reviews October 2004, p. 942-964, Vol. 17, No. 4
Creeping into your Head - A Brief Introduction to Microglia - A Review from the Science Creative Quarterly
"Immune Scavengers Target Alzheimer’s Plaques". Retrieved on 2007-05-09. - from Harvard University