Answer:
The blood-brain barrier (BBB) is formed from processes of many endothelial cells. Many substances cannot enter the BBB due to the tight junctions and the high passage selectivity of these cells.
Explanation:
A researcher named Paul Ehrlich would introduce a dye into an animal's blood supply and later observe that the only point that was not stained was the central nervous system, and specifically the brain. The reason for this had to be related to a protection system that surrounded that area as if it were a membrane.
Later, another researcher, Edwin Goldman, would try the reverse process by staining the cerebrospinal fluid, observing that the only colored parts corresponded to the nervous tissue. These experiments reflect the existence of something that produces a high level of blockage between the nervous system and the rest of the organism, something that years later would be called the blood-brain barrier by Lewandowski and explored by a large number of experts.
