Microglia are the resident immune cells of the central nervous system that survey the brain for damage and infection.Microglia are the resident immune cells of the central nervous system, which includes the brain and spinal cord. They are a type of glial cell, meaning they are not neurons but support and protect them. Unlike immune cells in the blood, microglia are born early in development and take up permanent residence in the nervous system, where they constantly survey their surroundings for signs of injury, infection, or dead cells. They make up about 10–15% of all cells in the brain. When they detect a problem, they can change shape, move toward the trouble spot, and release signaling molecules that trigger inflammation. They also help sculpt the developing brain by pruning away unused synapses, the connections between neurons. In this way, microglia are both caretakers and defenders, involved in normal brain function as well as in disease.
Microglia were first described in the 1910s by the Spanish neuroscientist Pío del Río Hortega, who used a silver staining method to visualize them in brain tissue. He named them 'microglia' to distinguish them from other glial cells. Their immune function was recognized later, and in recent decades they have become a major focus of research in neurodegenerative diseases.
Microglia originate from primitive macrophages in the yolk sac during early embryonic development and migrate into the brain before the blood-brain barrier forms. Once there, they extend and retract fine processes to sample the local environment. They carry receptors that recognize molecular patterns associated with pathogens or damaged cells. When a receptor binds its target, the microglial cell becomes activated: it retracts its branching processes, takes on a rounded shape, and moves toward the source of the signal. It can then engulf and digest debris or microbes through phagocytosis. Activated microglia also release cytokines and other immune molecules that can promote repair or, if excessive, cause damage to nearby neurons.
Microglia are studied as key players in Alzheimer's disease, Parkinson's disease, and multiple sclerosis, and drugs that modulate their activity are being tested to reduce neuroinflammation and slow neurodegeneration.
Microglia are not just passive defenders; they actively prune synapses in the healthy brain, and their dysfunction has been linked to conditions like chronic pain and even some psychiatric disorders.