Your brain is privileged—immune privileged.  It’s also not, though.  The term “immune privileged” refers to a part of the body that is inaccessible to the immune system.  One important job of the immune system is to run an almost constant surveillance of the body to ensure that intruders such as bacteria, viruses, and fungi are eliminated before they can cause an illness.

Our central nervous system, consisting of the brain and spinal cord, has a specialized type of cell known as microglia.  Although some of you might be familiar with neurons and how they communicate with one another to send messages throughout the body, microglia do not directly participate in the relay of these signals.  The term “glia,” used in neuroscience to describe a collection of non-neuron cell types, means “glue” in Latin.  Therefore, microglia can be thought of as part of the “glue” that supports the business end of our nervous system.  There are other cell types that make up the glia, or “glue,” but microglia are the only immune cell type.

Microglia roam the brain literally eating (the scientific term is “phagocytosing”) any debris or pathogen that could cause harm to healthy cells.  Microglia are the only immune cells that are native to the brain.  This means that other types of immune cells common in the rest of the body (i.e. white blood cells) cannot enter into the brain tissue thanks to the blood brain barrier.  While the skull and backbone both do a great job at protecting from injury, the blood brain barrier is essential when it comes to protection from harmful invaders.

The blood brain barrier is the term used to describe the blood vessels that are scattered throughout the brain.   This very important structure has two main jobs; the vessels deliver oxygen and nutrients to the brain tissue, just like the rest of our blood vessels.  These vessels also act as a literal barrier between pathogens and other cell types and brain tissue.  In this way, the blood brain barrier can be thought of as a moat.  In the case of a healthy, non-inflamed brain, the bridge to cross the moat is very selective as far as what can cross.  In most cases, only oxygen and very small molecules with a specific chemical makeup can cross.  When the barrier is compromised, as is the case in several different neurological diseases, so is the brain’s first line of defense.

What Happens when Foreign Cells Cross the Blood Brain Barrier?

When the structural integrity of the blood brain barrier is weak or destroyed, there are a number of dangerous events that can occur.   Interestingly, some species of bacteria have counter defenses, or natural ways of slipping through the blood brain barrier, even if the barrier is perfectly intact.  For example, bacterial meningitis and syphilis (yes, that syphilis—if left untreated for years, the bacteria causing the disease can migrate to the brain and cause a very serious disease known as “neurosyphilis”) are two diseases that can occur in people with a normal blood brain barrier.  However, a “leaky” barrier can contribute to other neurological infections such as human immunodeficiency virus (HIV), which researchers think can get through a weak barrier to cause a very serious form of acquired immunodeficiency syndrome (AIDS).

In other cases, a leaky blood brain barrier can invite what are known as “infiltrating immune cells” into the brain tissue.  Put simply, this means that different types of white blood cells are exposed to different types of glia (or “glue”) and neurons.  Some of these unwanted cells can cause almost immediate inflammation, meaning that they create and amplify signals that invite even more unwanted infiltrators.  Together, these cells create what is known as “neuroinflammation,” or an immune response within the brain (or spinal cord).  Several neurological diseases such as Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis are stimulated or made worse by a leaky blood brain barrier that opens the door for an unchecked inflammatory response.  The mechanisms that cause the breakdown of this barrier, however, are still not very well understood by researchers and doctors.

Multiple sclerosis is an autoimmune disease (a disease that occurs when immune cells damage healthy tissue as if it were an invading foreign substance) of the central nervous system.  On a basic level, it is caused when immune cells called  “T cells” migrate across the blood brain barrier and degrade a very important part of the neuron known as “myelin.”  Myelin is a fatty substance that is wrapped around some neurons (a big portion of these are motor neurons, which signal movement throughout the body).  It functions to “insulate” neurons to allow for quick and efficient relay of important cues to the body, similar to how electrical wires are covered with an insulator.  This insulator is produced by a different cell type (not microglia) that makes up the “brain glue.”  When myelin is degraded by T cells, neurons that are wrapped in myelin also become “leaky”, meaning that the electrical signal running through the neuron can literally leak out, slow down, or become significantly dampened.  This is why one telltale symptom of multiple sclerosis is a diminished sense of balance.

Leakiness of the blood brain barrier, however, is just one of many causes of inflammation in the brain (no one has ever accused neuroscience of being a straightforward study).  Many neurological diseases can be worsened or progressed by inflammatory activity in the brain.  Even if the blood brain barrier does not allow unwanted intruding cells, some of the non-microglia “glue” can cause an inflammatory response even though they are not technically immune cells.  For example, astrocytes (translating literally to “star cell” because of their shape when viewed under a microscope) are a type of glial cell.  Their main functions include providing structural support to the neural network, similar to a scaffold.  Not only do these cells contribute largely to the “glue-like” nature of glial cells as a whole, but they can also act as a physical guide for developing neurons and active synapses (the microscopic space between two communicating neurons).  Do not be fooled, however, as astrocytes can amplify inflammation and communicate physically and chemically with microglia to ultimately play a significant role in inflammation.

Why Should I Care?

Even if you have a healthy nervous system, there is a great benefit to knowing the potential risks for neurological disorders!  For example, some behaviors that can raise an individual’s risk of neurological disease are substance abuse, decreased blood circulation (this includes living a sedentary lifestyle/lack of exercise), and insomnia.  Of course, there are a lot of other complication factors, like genetics, that determine the true risk for neurological disease for a given individual.

The Appalachian region of the United States of America has a particularly high instance of neurological diseases, especially stroke and Alzheimer’s.  Public health experts believe this to be a result of multiple factors, one of these being an overall lower socioeconomic status, meaning this region of the country has a higher than average poverty rate.  Most Appalachian communities are also relatively small and isolated from larger cities by both distance and terrain, resulting in a relatively low rate of convenient access to healthcare.

Knowing the risks of neurological disease can help you be aware if you or someone around you is at an increased risk.  As stated before, these factors alone cannot determine precise risk.  Knowing the basic risks and how to reverse them can help to empower individuals to look out for their health and the health of others.

Disclaimer: The author of this article is not a licensed physician, therefore, the information presented should not be used to diagnose or treat any medical condition(s).  Please consult a licensed physician for medical advice and/or treatment.