What is the evolutionary reason for the blood brain barrier? It’s absolutely fascinating. No other organ has this, but you would expect through randomness that it would exist outside of the brain.
Many potential neurotoxins are circulating in our blood, including those from endogenous sources such as metabolites or proteins, or exogenous ones such as xenobiotics ingested in the diet or otherwise acquired from the environment.
I would speculate that other areas of the body haven't developed similar protections because their structures are not as vulnerable to stuff that's floating around in your bloodstream.
The eyes have a similar barrier. I think the current belief is that your body wants to make sure that your T-Cells cannot accidentally nuke your brain and eyes. Those don't grow back and are vitally important, so they have to be protected from the "Nuke it from space" approach of the immune system.
Nit: I’d rephrase to say that your body has no opinions about T-cells. But people who had proto-BBBs survived better than those who didn’t, AND T-cells would nuke the central nervous system from space.
The central nervous system has an extremely delicate chemical environment where routine disturbances like inflammation would induce swelling and destruction of delicate neural pathways. In fact one of the primary roles of the blood-brain barrier is to ensure that any type of “growth” or “organ repair” is extremely retarded - any changes to cells happen very slowly compared to the rest of the body. This is one of the reasons brain damage is so slow to repair, the whole nervous system has a very prescribed way that it grows as a fetus and childhood/youth and then it puts the brakes on everything and only allows very small changes from then on.
