The blood–cerebrospinal fluid barrier (BCSFB) is a fluid–brain barrier that is composed of a pair of membranes that separate blood from CSF and CSF from brain tissue.[1] The blood–CSF boundary at the choroid plexus is a membrane composed of epithelial cells and tight junctions that link them.[10] The brain–CSF boundary is the arachnoid membrane, which envelops the surface of the brain.[10]