This a dynamic interface between the brain and the body, which regulates what substances enter the brain parenchyma. The blood brain barrier is made of specialized endothelial cells and their surrounding astrocytic foot processes. The endothelial cells in the brain vasculature are non-fenestrated, lack intercellular clefts, and pinocytic vesicles. Furthermore, they contain much more mitochondria than normal systemic endothelial cells, and this is to supply energy to the multiple energy-dependent transporters in the BBB. In general, though, the more lipid soluble a substance, the easier it passes through the blood brain barrier.
Referred to as: BBB
The blood vessels of the brain (and the retina) are much more impermeable to large molecules (like antibodies) than blood vessels elsewhere in the body. This has important implications for the ability of the organism to mount an immune response in these tissues, although the basis for the difference in endothelial permeability is not well understood.