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What is blood brain barrier?

The blood brain barrier (BBB) is an essential component of healthy brain function. As the name suggests, specialized cells create a highly selective barrier that separates the blood and blood-borne materials from entering the brain.

This barrier is necessary because many blood-borne materials are neuro-toxic and can inhibit or damage normal brain function.  

In addition, the BBB also prevents potentially harmful immune cells from entering the brain and causing tissue damage.  
The BBB is comprised of a layer of cells on the vessel wall called endothelial cells surrounded by cells called pericytes and astrocytes from the brain.
This close contact of these cells creates an environment in which there is a physical barrier between the blood and brain tissue.
This physical barrier prevents blood-borne material from seeping into the brain.
Only highly selective material that is needed for the brain or required to be passed are allowed to go through. 
However, during disease states like multiple sclerosis, the BBB can be damaged and result in harmful blood-borne material and immune cells entering the brain and causing damage.  
Jennifer Linden, PhD

Weill Cornell Medicine Multiple Sclerosis Center 1305 York Ave., Second Floor New York, NY 10021