Re: On the other side of the blood-brain immune barrier

The blood-brain barrier refers to the inability of blood-derived 
chemicals to pass into the brain. The endothelium of the capillaries 
of the brain forms an inpermeable barrier to chemicals found in the 
blood. However, cells of the immune system can cross the barrier 
through a process called diapedesis or extravasation. The cells of 
the immune system can actively squeeze through the junctions 
between the endothelial cells to enter the tissue. In fact, it is probably 
the immune system that carries viruses and intracellular bacteria into 
the brain that causes some infections in the brain.

Yes, in addition, the brain has its own immune components and 
scavenger cells. Some of these cells are called pericytes and 
microglial cells (specific to the brain). These cells are not specific to 
particular antigens, such as T and B cells, but are part of the 
non-specific immune response system.