Question

what type of cells form the blood - brain barrier?
white blood cells that create a protective layer around the brain.
tightly packed cells that line the blood vessels in the brain.
loosely packed cells that line the blood vessels in the brain.
cells that are found in high concentration in the cerebrospinal fluid.
neurons that form a barrier around the brain.

Explanation:

The blood - brain barrier is formed by tightly packed endothelial cells that line the blood vessels in the brain. These cells have specialized tight junctions that restrict the passage of substances from the blood into the brain tissue. White blood cells are part of the immune system and not the main component of the blood - brain barrier. Loosely packed cells would not create an effective barrier. Cells in cerebrospinal fluid are not involved in forming the blood - brain barrier, and neurons do not form this barrier.

Answer:

Tightly packed cells that line the blood vessels in the brain.