Once viewed merely as a producer of the cerebrospinal fluid (CSF) bathing the brain and spinal cord, the choroid plexus is now known to be a key player in brain development and immunity. These fronds of brain tissue, located in the CSF-filled brain cavities known as ventricles, secrete instructive cues into the CSF to regulate brain development. They also function as an important barrier between the brain and the rest of the body.