No immune cells in the brain?

The human body protects itself from diseases caused by bacteria and viruses through our very own immune system. There is a large diversity of immune cells within our bloodstream, each having its own distinct way of protecting our health. Having a broad range of immune cells in our body serves as our first line of defense against pathogens and diseases. Following that would be the use of appropriate medicine if the condition doesn’t get better.

 

We might assume that our immune cells are present in all places of the body since it’s circulated through the bloodstream. But that is not the case! Some sites of our body can be referred to as being ‘immune privileged’. Places such as the brain, eyes, uterus, testes are observed to have the ability to tolerate pathogens and foreign cells, without eliciting an inflammatory immune response.

 

Does that mean there are no immune cells there?

 

Indeed! Among the sites mentioned above, immune cells cannot freely infiltrate that area. One common characteristic that the 4 sites have is that their tissue is different from other organs in the body. These tissues are specialized in the way that they are not able to be regenerated again if it gets damaged, that could possibly be due to immune response. In contrast, our skin tissue is able to regenerate pretty quick after experiencing a small cut.

 

Brain

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The brain in particular is a critical part of our body’s functioning and survival. It is immune privileged, and there are no immune cells circulating in and out of the brain. Instead, the brain has microglia, which serve as its immune defense instead of our immune cells from the bloodstream. In between the brain and our blood circulation, there is the ‘blood brain barrier’ (BBB). This barrier consists of very tight junctions that prevents any particle from coming in and out of the brain! This barrier is one of the reasons why immune cells are not present in the brain.

 

However, the lack of immune cells in the brain is not necessarily a bad thing! Some of neurological diseases are actually caused by extensive stimulation of the immune response. So, it’s good in a way that there is a barrier to protect the brain from being attacked by the body’s own immune cells.

 

On another note, in some circumstances when there is a pathogen present the microglia in the brain serve as its first line of immune defense. The microglia are able to identify the pathogen, and call for back up from the body’s extensive range of immune cells! Microglia are able to signal to particular receptors for reorganization of the barrier, which leads to loosening of the tight junctions. This gap will allow immune cells from the bloodstream to pass through the blood brain barrier, and infiltrate the brain to attack the pathogens present.

 

It is not only fascinating that the brain has a barrier to protect against immune cells, but also the way it can reorganize the structure of this barrier to allow selective infiltration of immune cells whenever needed.