Meninges
The blood-brain barrier in the meninges' blood vessels protects the brain from most pathogens (things that cause infection).  Because of this, infections in the brain are not very common.  This is important, because antibodies - which the body's immune system makes to fight off infections - cannot get past the blood-brain barrier into the brain.  Neither can most antibiotic medications.  This means that when a person does get a brain infection, the body usually cannot fight off the infection on its own, and most of the medications that doctors use to kill infections cannot get into the brain to kill the pathogens inside.