BB isolates the central nervous system (CNS = brain and spinal cord) from even the blood and fluids that bathe all other tissues in the body. The cells forming the walls of blood vessels in the cental nervous system are sealed together so tightly that cells and molecules in the bloodstream that freely pass into tissues elsewhere in the body are unable to cross into brain tissue. This deprives the CNS of access to immune system cells that do not penetrate the CNS. (Fields, R. Douglas, PhD. The Other Brain. p 41-43. NY:Simon & Schuster, 2009.)
Zonulin is a protein made by the human body that appears to impact the development of auto-immune diseases as well as those that involve blood-barrir dysfunctions such as multiple sclerosis, HIV infection, and brain tumors. Zonulin acts as a gate-keeper for brain and body tissues. It can open junctions between cells known as zonula occludens, allowing some subsgances to pass through that ordinarily would be stopped. This phenomenon has been shown to occur not only in the intestines (zonulin levels are higher in individuals with celiac disease) but also in the blood-brain barrier. Korn, Dana, with contributions by Michelle Maria Pietzak MD, and Alessio Fasano MD. Wheat Free - Worry Free. p 333-340. CA:Hay House, 2002.)
Refer to Microglia in Glial Cells – the Other Brain for additional information.