Everybody knows about the immune system, right? You get an infection and your body mobilises its army of immune cells and fights off the invaders. You get a fever, feel awful for a while, then things get back to normal. However, in the brain, it doesn’t work that way. In immunology, the brain is called privileged. This doesn’t mean its parents had money and it went to a better school than you; it means it’s excluded from the normal immune response. In this post, I introduce you to the wondrous world of neuroimmunology, the study of the immune system in the brain. Hopefully this will whet your appetite for a longer article exploring this more in depth, which will be coming out later.
The brain is separated from the rest of the body and the blood circulation by the blood-brain barrier, allowing the brain to be selective with what it lets in, and protecting it from infection. However, this also means the usual immune cells normally can’t get in. Luckily for us, the brain has its own immune cells, microglia. These cells are continuously patrolling your brain for anything that shouldn’t be there, and will attack and clear out any infections. Although this is an important role, unless you’re unlucky enough to contract something like meningitis, most neuroimmunological processes actually do not involve infection.
So why should you care about this? You should care because neuroimmunology is involved in pretty much every brain process and disease investigated so far. The most obvious ones are autoimmune diseases of the central nervous system, such as multiple sclerosis, where the immune system incorrectly recognises your own cells and proteins as something foreign to your body and attacks them. However, there are many more diseases with neuroimmunological involvement.
In neurodegenerative diseases such as Alzheimer’s or Parkinson’s, microglia are activated and clear out the many dying cells. However, the signalling factors microglia secrete can cause more neuronal dysfunction and cell death, creating a vicious circle. Activation of immune cells has also been found in many psychiatric diseases such as chronic depression and schizophrenia, but we don’t yet have a clear idea of their roles in these conditions. However, the immune response in some of these diseases is now starting to be studied as a target for potential new treatments.
It is easy to view the immune system as something that only kicks into action during infections or disease. It is, in fact, working all the time and is part of the brain’s normal development and maintenance. Neurons continuously make new connections and lose old ones. Microglia, the immune cells of the brain, are responsible for clearing out old and faulty connections, ensuring your neurons connect the way they should. The immune system is also closely involved with the formation of new neurons. These roles can again be affected by disease. For instance, my own research has shown that neuroinflammation in chronic epilepsy may be responsible for lower numbers of new neurons and the memory problems patients suffer from.
Neuroimmunology is still a rapidly expanding field, and we are continuously finding new roles for immune cells and factors in the brain. In fact, only last month, research suggested that immune factors are involved in the regulation of social behaviour in a range of animals. (Note: you could try this as an excuse for being antisocial next time you have a cold. Don’t blame me if it doesn’t work though.) Having now hopefully convinced you that you should care about neuroimmunology, stay tuned for a longer post exploring its roles in detail!