The brain’s resident immune cells are microglia. They constantly survey the tissue with motile processes, ready to respond to injury, infection, or the presence of abnormal proteins. In their resting state they also participate in synaptic pruning and the refinement of neural circuits during development and adulthood.

When activated, microglia can phagocytose debris, release inflammatory cytokines, and recruit other immune cells. The response is usually protective, yet chronic or excessive activation contributes to the pathology of neurodegenerative diseases, stroke, and traumatic injury. Microglia originate from yolk-sac progenitors and form a self-renewing population distinct from peripheral macrophages.

Modern genetic and imaging tools have revealed that microglia are highly dynamic and heterogeneous. Their dual roles in defense and in normal brain maintenance make them important targets for therapeutic intervention in a growing list of neurological conditions.

A single microglial cell can contact multiple synapses while simultaneously monitoring the local environment for threats.