If someone can find newer reports showing that this information is no longer holding true, please do share!! But, from what I've learned over the years:
www.lymeneteurope.org/info/the-complexities-of-lyme-diseaseBy Tom Grier (Microbiologist that had Ehrlichia and Lyme)
...
"It appears that Borrelia are able to change their surface antigens many times, and can do it quickly. In one study by Dr. Andrew Pachner MD, he infected mice with a single strain of Borrelia burgdorferi. After several weeks, he was able to isolate two slightly different forms of the bacteria. The bacteria from the bloodstream was attacked and killed by the mouse's immune sera, but the bacteria isolated from the mouse's brain was unaffected by the immune sera. The bacteria isolated from the mouse's brain had a new set of surface antigens. It appears that contact with the CNS caused the bacteria to change its appearance. Since the brain is isolated from the immune system and is an immune privileged site, the bacteria became its own separate strain. "
"What happens when the infection gets to the brain? In the case of Lyme disease, every animal model to date shows that the Lyme spirochete can go from the site of the bite to the brain in just a few days."...
"Since the brain has no immune system, it prevents infection by limiting what can enter the brain. The capillary bed that surrounds the brain is so tight that not even white blood cells are allowed to enter. Many drugs can't enter either, making treatment of the brain especially hard. For the first ten days of a Lyme infection, the blood brain barrier is virtually nonexistent. This not only allows the Lyme bacteria to get in, but also immune cells that can cause inflammation of the brain.
I can spend time looking for more, but I would encourage you to do have a look for yourself. The information I found was from the late 90's I believe - so more current research could prove it wrong, although I haven't seen any yet.