The brain is considered the most complex living structure known in the universe. This organ is responsible for controlling all bodily functions ranging from heart rate to motor functions. Given the complexity and importance of the brain, it is vital that proper defenses are maintained, one of which is the blood brain barrier. The blood brain barrier (BBB), as the name implies, serves as an anatomical barrier against invading pathogens by blocking their entry into the brain. Think of it as the Great Wall of China that was built in order to protect the Chinese empire against invasion by nomadic tribes. In this analogy, the brain would be considered the Chinese empire. Similar to the bricks and stones making up the Great Wall, tight junctions present between the endothelial cells of the blood vessels make up the BBB and they function to allow passage of only certain molecules such as oxygen. Even molecules such as glucose must undergo an active transport mechanism in order to pass through the BBB. The central nervous system (CNS) is considered an immunoprivileged organ in that CNS antigens are not accessible to immune cells in the periphery. Likewise, peripheral immune cells and pathogens cannot easily penetrate the BBB.
During the course of various neurological diseases such as multiple sclerosis (MS), vascular dementia, and stroke, this great wall becomes compromised thereby triggering the infiltration of immune cells into the CNS. This process in turn causes inflammation. Studies on animal models of these diseases aim to unravel the mechanisms by which certain immune cells breach the BBB (Simka, 2009). Unraveling such mechanisms provides therapeutic targets for future investigations in the hope of finding an effective treatment.
