Type O negative blood (red cells) can be transfused to patients of all blood types. It is always in great demand and often in short supply. Type AB positive plasma can be transfused to patients of all other blood types. AB plasma is also usually in short supply.
Blood-brain barrier. A blockade of cells separating the circulating blood from elements of the central nervous system (CNS); it acts as a filter, preventing many substances from entering the central nervous system.
The answer is because of the blood-brain barrier. This layer of cells keeps your brain safe from pathogens and other toxins. It is composed of brain cells and blood vessel cells. It plays a crucial role in protecting the most important organ in your body: the brain. The blood-brain barrier is like the moat outside a castle, preventing harmful invaders from entering.