The blood-brain barrier is the barrier between plasma and brain cells formed by the walls of brain capillaries and glial cells, and between plasma and cerebrospinal fluid formed by the choroid plexus, and these barriers prevent certain substances from entering brain tissue from the blood. A variety of solutes from the blood enter brain tissue from the brain capillaries, some more easily and some more difficult. For the role played by the blood-brain barrier, it is really mainly the ability to block certain substances from entering the brain tissue. However, if there is inflammation of the brain parenchyma, then it is possible that certain antibiotics may have difficulty reaching the brain tissue, so the blood-brain barrier is a general term for the three conditions of blood-brain, blood-cerebrospinal fluid and cerebrospinal fluid-brain. Compared to capillaries in other tissues and organs, brain capillaries, as well as the adjacent areas, have some distinctive structural features. Cerebral capillaries lack the pores that are normally found in capillaries, or these pores are few and relatively small.