Several routes exist for immune cells and neurons to communicate, though T cells rarely come in direct contact with neural tissue. This communication can happen as cerebrospinal fluid (CSF) flows from the space surrounding blood vessels deep in the brain into neural tissue and back out again. As an animal learns new information, changing neural circuits can release signals to which the immune system responds. The immune system in the meninges, the spongy membranes that separate neural tissue from the skull, also monitors CSF coming from the brain for signs of infection or injury.
The meninges’ innermost layer, the pia mater, lines the perimeter of the brain, separating neural tissue from the surrounding fluid and tissue. But gaps in the thin, fibrous tissue allow blood vessels to extend deep into the brain.
Along blood vessels in the brain, a tightly packed layer...
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