Perhaps the singularly striking feature of synaptic pruning is that it uses an immunological mechanism to eliminate connections between neurons. Macrophages in the immune system phagocytose—eat—pathogens and cellular debris. Microglia in the brain use some similar proteins and processes to mark synapses that are to be nibbled—except, rather than ingesting pathogens, they ingest the bits of a neuron involved in the synaptic connections. It’s yet another captivating instance of repurposing: the very proteins and pathways that are used to clear pathogens in the body have been rejiggered to
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