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Unformatted text preview: are involved in synaptic pruning and programmed cell death. They also express cytokines, the
signaling molecules that serve as messengers between cells and are a key component of the body's inflammatory
response. Cytokines are important for the development of basic brain structures from blood vessels to axons. They are
also involved in regulating cognition and mood, Bilbo says.
She explored the brain's immune cells by infecting infant rats with the bacterium E. coli four days after birth, a
developmental period comparable to the third trimester of pregnancy in humans. The young rats recovered fully from the
infection, and as adults they performed as well as control rats on tests of memory and cognition. But their early infections
had left their mark.
The rats' microglia had been "primed," Bilbo says; in essence, the cells had been put on high alert for future infections.
When the rats experienced a second infection—what she calls a "second hit"—around the time they were learning a new
task, they showed profound memory impairments for that task. The p...
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