The Immune System Protects the Brain by Staying Away
The human immune system has many different cell types that patrol our bodies constantly and keep us healthy. The brain is a particularly important organ for human functioning - does this mean there are thousands of immune cells monitoring every nook and cranny of our wrinkly brains? Not exactly.
Imagine you were bit by an ant, and you see the tiny bump left behind by its pincers. Your body reacts by rushing blood to the site of the wound, causing some swelling, itching and redness. This blood carries an army of immune cells, which gets rid of any dangerous things from the ant bite. This process is called inflammation. Since our skin is generally stretchy and can change shape, a little inflammation from the bite isn’t really an issue. However, that isn’t true for all parts of the body. The brain is tucked tightly into our skull, so any inflammation in our brain would put pressure on the skull and damage precious brain cells! So how does the immune system protect the brain?
Long story short, the immune system patrols the brain differently from how it patrols the rest of the body. The human body has a “blood-brain barrier” (BBB) - a physical separation between brain cells and blood vessels that only allows certain chemicals, called molecules, to enter and exit our brain [1]. The BBB lets good molecules like water and oxygen into the brain to help our brains function, while stopping any harmful substances from entering the brain. It also keeps immune cells away from the brain, so inflammation rarely ever happens!
But, sometimes inflammation of the brain is possible - in fact, many serious neurological disorders are caused by immune cells entering the brain [2]. When this happens, inflammation in the brain causes lots of problems that make it harder for our brain and nervous system to function.
So if the immune cells cannot enter the brain directly, how do they protect the brain from disease? Interestingly, the blood vessels lining the outside of the brain are highly concentrated with tons of different immune cells found throughout our body [3]. These cells watch over everything coming in and out of the brain, and help the BBB maintain a healthy environment. If they notice any concerns, they can safely enter specific parts of the brain to stop any threats, without causing too much inflammation.
However, we don’t know how each type of immune cell works in the brain. We don’t know a lot about how things like age, diet and even bacteria from other parts of the body effect the way our brain and immune system communicate. What we do know is that immune functioning near the brain is intensely regulated and controlled.
Essentially, our immune system monitors the brain from the outside, like guards on the castle wall, armed and protecting the royal family tucked away inside.
References
1. Dotiwala AK, McCausland C, Samra NS. (2023). Anatomy, Head and Neck: Blood Brain Barrier. StatPearls Publishing
2. Bhandari T. (2022) How does the immune system keep tabs on the brain? Washington University School of Medicine in St. Louis
3. Kwon D. (2022) Guardians of the brain: how a special immune system protects our grey matter. Nature Communications 606, 22-24
Edited by Alexandra Fink
