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> The immune system  monitors the brain from without:
> 
> https://www.quantamagazine.org/how-the-immune-system-protects-the-brain-20210428/
> 
> "Rustenhoven and collaborators have identified how evolution achieves a balancing act, limiting the dangers of immune responses in the central nervous system while still providing protection from disease. The researchers reported recently in the journal Cell that the immune system operates from a distance to constantly inspect the brain for signs of trouble. Immune cells, rather than making themselves at home throughout the brain itself, patrol the sidelines until they detect a threat.
> 
> “'Immune surveillance of the brain takes place. It’s absolutely normal, like in any other tissue,” said a coauthor, Jonathan Kipnis, in whose lab at Washington University the research took place. “The only exception is that instead of this happening within the tissue, the brain pushed all its immune activity to its borders.”
> 
> "Using several kinds of imaging and tracing, the researchers tracked the cellular choreography that makes up this surveillance system. They saw that antigens — foreign substances, such as bits of pathogens — were washed from the brain in a flow of cerebral spinal fluid. This fluid streamed through a network of vessels that Kipnis’ lab identified a few years ago and swept the antigens along so that they accumulated in the back of the brain. Here, in the area around the dural sinuses — channels on the brain’s border that drain fluid out toward the body — the antigens came into close proximity to immune cells. “Everything’s actually highly concentrated in that one particular site,” Rustenhoven said.
> 
> "These vessels curving around the back of the brain proved to be a hub of immune system activity. The researchers tracked antigens and other substances crossing the arachnoid barrier, an obstacle that’s known for its impermeability but which, they found, leaks in this particular area. Immune cells are waiting there. When these cells find a worrisome antigen, like one that suggests disease, they initiate a chain reaction that creates an immune response.
> 
> ***
> 
> "Tumors close to the ventricles flushed lots of antigens out to the edges of the brain, which caused a strong immune response. The more distant tumors flushed fewer antigens, which caused a weak immune response, one that the tumor could overcome.
> 
> "In other words, the immune system’s surveillance of the brain is a bit spotty — a downside of the balancing act that evolution has achieved. “Maybe this is a necessary compromise,” Rustenhoven said."
> 
> Comment: The so-called blood brain barrier keeps most chemicals in the blood away from the brain. On the other hand immunity had to be present. teh result  is another complex design.