Dr. Richard Ransohoff (Formerly of the Cleveland Clinic and Biogen)
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​Meet the Microglia -- 2016 Talk - University of Washington School of Medicine, Neurology Grad Seminar Series

Richard Ransohoff provides a superb history of the new microglia field since the 2005 publication of 2 photon microscopy based video of microglia motility in the living brain.

Boiled down to its essence, the talk argues that microglia must now be understood as brain cells essential to physiological brain function, not immune cells which happen to be in the brain to provide host defense. 

What are these functions? We are at the beginning of understanding them, but microglia are clearly essential to sculpting  the neuronal architecture and synaptic landscape of the brain as it  develops, and they remain intimately involved in regulation of synapse function later in life. 

When microglia react immunologically in the context of infection, injury and disease, they can't perform these  functions  necessary to physiological brain.