What Are Immune Privileged Organs?
The immune system can be both harmful and helpful to your overall health. When it’s being helpful, it uses a set of reactive processes that help to identify, locate, and fight off pathogens and other substances known as antigens that can cause disease or illness. When it works against you, however, it can mistake your own healthy tissues for pathogens and end up damaging the body in its quest to protect it.
In some cases, the immune system can be sensitive to non-harmful substances such as pollen, pet dander, or even dust and that, too, sets off an immune reaction. To say that a lot goes into the body’s immune system and its reactions would be an understatement!
While the majority of the body’s organs are at the mercy of the immune system when it comes to the presence of antigens, there are some areas that may not be. Those areas are referred to as immune privileged. But what is immune privilege, exactly, and what parts of the body fit into that category? Read on to learn the answer to the question: what are immune privileged organs?
What does immune privilege mean?
Typically, antigens set off inflammation because that is how the body identifies where an antigen is located so that it can send the proper fighter cells to the area. Inflammation is also designed to contain a pathogen in one area so that those cells have time to travel there before it spreads to other areas of the body.
As mentioned above, immune privilege refers to some areas of the body that are essentially immune (pardon the pun!) to an immune reaction. They do not trigger any sort of response when they come into contact with an antigen.
How does immune privilege work?
Immune privilege is a simple process with various underlying causes. It essentially works by failing to alert the immune system to take action against an antigen or other foreign substance, even when one is present. Areas in the body that are immune privileged can do this for a variety of reasons. The first has to do with their structure.
In immune privileged organs, there is a lack of traditional lymphatics. Lymphatics are part of the lymphatic system, which is a set of small vessels that connect lymph nodes. Lymph fluid passes through the vessels and is filtered through the lymph nodes. Lymph fluid is the clear liquid that helps immune cells travel throughout the body to help fight off infection.
When an organ contains lymphatics, it is able to alert the immune system when there is a pathogen present. However, when an organ lacks traditional lymphatics, that process is hindered, and thus the immune system isn’t alerted in the same way to send immune cells to fight off whatever foreign substance has made its way into the organ.
Immune privileged organs also lack something called histocompatibility complex class II expressing antigen-presenting cells. This category of cells groups together molecules that are typically found on specific cells designed to initiate an immune response in the face of a threat. Without these molecules, no immune response can be triggered.
There are also some physical barriers that prevent immune privilege sites from triggering the immune system.
What are immune privilege sites?
There are various sites on the body that are less prone to evoking an immune response than others. You can think of these sites as part of the collective of the body but separated (much like a secret society would be from the rest of the community!). The parts of the body that are typically considered to be immune privileged are the brain and central nervous system, the eyes, the testicles, and the uterus.
That isn’t to say that no immune reaction can be sparked if a pathogen makes its way into these sites, just that it is less likely. Some other areas of the body, such as the cartilage or hair follicles, may also possess some immune privilege when it comes to setting off an immune reaction.
Originally, the aforementioned sites were found to have immune privilege because of the way they reacted to grafts. Grafts are living tissue transplants that move some live tissue from one area into another. In most areas of the body, a graft may be rejected depending on the immune reaction, but in immune privileged areas, the chances of rejection are substantially lower.
Is the liver an immune privileged organ?
While the eyes, brain, uterus, and testicles are all considered immune privileged sites, there are other tissues in the body that can possess similar abilities to hinder or impair the immune response so that it doesn’t pass on the inflammatory message. One such tissue that can act as if it’s immune privileged is the liver.
While the liver wasn’t originally considered to have this special ability, the possibility has been seen in research that the liver can act similarly to other organs that have immune privilege, even though it possesses itself a large number of immune cells.
When it does this, the process is more often referred to as an immune tolerance rather than privilege. Immune tolerance isn’t the exact same thing as privilege, but it can act in a similar way by negating an immune response. However, tolerance occurs only with specific pathogens.
While the immune system does act as a helpful defense system for our health, it can also be set off unnecessarily. Not all tissues and organs mitigate this through immune privilege, but some can do so as a way to help prevent immune reactions where they are not needed.