It turns out that the developing brain treats unwanted synaptic connections in exactly the same way. Unwanted connections are “tagged” for destruction with immune system molecules called complement proteins. Microglia recognize this as a signal saying “eat me,” and engulf all the tagged synapses they come across. It’s now thought that microglia are responsible for synaptic pruning throughout the developing brain, as well as for the extensive pruning that occurs in adolescence
