The poor function of microglia, the brain's immune cells in individuals exposed to early life adversity (ELA) promotes aberrant responses to stress in adulthood that may be linked to mental illness, according to a new study. During brain development, microglia prune unnecessary synapses resulting in the formation of refined, functional circuits. Disruption of that process leaves too many synapses, changing the behavioral and hormonal responses to further stresses later in life.