Microglia are the resident immune cells of the human brain and central nervous system.
They constantly survey brain tissue with mobile processes and can change shape when they detect injury, infection, or cellular debris. Activated microglia may engulf damaged material through phagocytosis and release signaling molecules that influence inflammation and repair.
Microglia arise from early yolk-sac progenitors, unlike most blood-derived immune cells. They populate the developing central nervous system before the blood–brain barrier is fully established and can maintain themselves locally for long periods. Their roles extend beyond defense: they also help shape neural circuits by pruning some synapses during development.
Astrocytes and oligodendrocytes are important glial cells, but they are not the brain’s resident immune cells. Astrocytes support neurons and regulate the chemical environment, while oligodendrocytes produce myelin around central nervous system axons.