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.
But these blood vessels are different from all the other blood vessels in our body. They are lined by a strange structure called the blood-brain barrier. It closely controls what is, and is not, allowed to leave the blood supply and enter the brain. It's your own personal customs and border security.