Harvard Mental Health Letter

Ask the doctor: What is the blood-brain barrier?

Q. What is the blood-brain barrier? I've heard that it may have something to do with psychiatric disorders. Is that true?

A. The blood-brain barrier is an inborn biological structure that protects the brain from potentially harmful substances circulating in the bloodstream. Although the word "barrier" may conjure up an image of a concrete highway divider or a brick wall, this barrier is not actually impermeable. Consisting of a tightly packed cell wall that is semipermeable, it is more of a border than a barrier, one that allows some substances to pass into the brain while blocking others.

This boundary around the brain has been recognized for more than a century, but only since 2000 or so have we learned more about how the blood-brain barrier actually functions. In 1885, Dr. Paul Ehrlich, an immunologist and Nobel laureate probably best known for discovering an early cure for syphilis, provided the first scientific evidence that some type of obstacle existed between the brain and circulating blood. In experiments with laboratory rodents, Dr. Ehrlich noticed that water-soluble dyes, injected into the limbs, stained organs such as the kidneys and heart, but not the brain.

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