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> The olfactory neurons have special protections to spare the  brain from viral infections:
> 
> https://medicalxpress.com/news/2020-06-microglia-olfactory-bulb-nose-brain.html
> 
> "'Airborne viruses challenge our immune system all the time, but rarely do we see viral infections leading to neurological conditions," said Dr. McGavern. "This means that the immune system within this area has to be remarkably good at protecting the brain."
> 
> "Additional experiments showed that microglia, immune cells within the central nervous system, took on an underappreciated role of helping the immune system recognize the virus and did so in a way that limited the damage to neurons themselves. This sparing of neurons is critical, because unlike cells in most other tissues, most neuronal populations do not come back.
> 
> ***
> 
> "'If a virus infects the processes of neurons that dangle within the airway, there is a chance for this virus to enter the brain, and ultimately cause encephalitis or meningitis," said Dr. McGavern. "We are interested in understanding immune responses that develop at the interface between nasal olfactory neurons, which end in the olfactory bulb, and the rest of the brain."
> 
> "Dr. McGavern's team was able to show that CD8 T cells, which are part of the immune system responsible for controlling viruses, are very important in protecting the brain after infection of nasal tissue. Using advanced microscopy, his group watched in real time how CD8 T cells protected the brain from a nasal virus infection.
> 
> "Interestingly, the CD8 T cells did not appear to interact directly with neurons, the predominately infected cell population. They instead engaged microglia, which are central nervous system immune cells that act a bit like garbage collectors by clearing cellular debris and dead cell material. When a viral infection occurs, the microglia appear to take up virus material from the surrounding environment and present it to the immune system as though they had become infected.
> 
> "In this way, infected olfactory neurons can "hand off" virus particles to microglia, which were then detected by the T cells. The T cells then respond by releasing antiviral molecules that clear the virus from neurons in a way that does not kill the cells. Because microglia are a renewable cell type, this type of interaction makes sense from an evolutionary standpoint.
> 
> "'The immune system has developed strategies to favor the preservation of neurons at all costs," said Dr. McGavern. "Here, we show that microglia can 'take the blow' from neurons by engaging T cells, which then allows the antiviral program to play out."
> 
> ***
> 
> "It is important to note that widespread infection of the olfactory sensory neurons, whether by the novel coronavirus, the virus used in this study, or any other similar virus, will likely disrupt our sense of smell. However, unlike other neurons in the central nervous system, these sensory neurons that begin in the nose and end in the brain are capable of regenerating after an infection is cleared.
> 
> "'The immune response we describe does not protect olfactory sensory neurons nor the sense of smell," explained Dr. McGavern. "This is not necessarily a long-term issue, because [b]those sensory neurons can be replaced once the virus is dealt with.[/b] What is critical is to protect the brain and central nervous system from encephalitis or meningitis—our sense of smell can often be repaired over time.'"
> 
> Comment: Another specifically designed system to protect the brain in which neurons can be reproduced quickly, while in general neurons are not reproduced except in the hippocampus.