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IMMUNE SYSTEM AND AGE: It is common knowledge that our brain starts becoming weaker as we grow older. Elderly people are more prone to cognitive disorders, proving the relationship between the immune system and age. Researchers have tried to identify the culprit behind all these issues in hopes to slow down the process of aging and maintain a healthy mind for as long as possible. However, a recent study, published in nature neurology in 2020 titled
“Neuroblast senescence in the aged brain augments natural kill cell cytotoxicity leading to impaired neurogenesis and cognition”
revealed that the culprit behind brain-related problems was none other than the brain itself.
OLD AGE IS A DISEASE
Old age has been seen as mankind’s greatest enemy, since ancient times. After power and money, immortality has been the most sought thing in the world.
The ancient Roman playwright Terentius had even declared“Old age itself is a sickness”. A team of researchers, in 2015, reinforced this by asserting that “It is time to classify biological aging as a disease.” Naturally, much research started being conducted to overcome this enemy. They identified many factors that aided wearing out of the body. these included environment, ethnicity, genes, and the combined effect of immune system and age.
IMMUNE SYSTEM IN OLD AGE:
The study published in nature basically revealed the main cause behind age-related problems. The brain became overactive and the cells in the brain started behaving out of ordinary.
The immune system and age are related because with time some anomalies start occurring. Before we dive into the details, here is the concise sequence of events. “Neuroblasts” are the cells of the brain. In elderly people, neuroblasts fall in a state of time-arrest. Then they start secreting some chemicals known as “cytokines” which are used to communicate between cells.
These chemicals then bring the Natural killer (NK) cells of the immune system to the brain. NK cells start “eating” the neuroblasts which lead to a decrease in brain functions. Identifying these affairs in the body opened up many possibilities by which researchers can increase human life span which is discussed later on.
WHAT CAUSES AGING:
This was a new insight into how the immune system and age were affiliated with respect to the brain. Along with why aged people suffered mind-related problems. However, scientists were already well aware of the immune system’s involvement in the aging process. Old age is a multi-factorial disease. As we grow older, our body starts going in a state of “inflame-aging”.
This occurs when there is low grade but permanent/long-term inflammation. Usually, inflammation is when the defense system (our immune system) gets ready to fight the infection. However, in some cases, there is no infection to fight. Then the hyperactive immune system starts damaging the organs. This helps in worsening the diseases such as cancer, diabetes, cardiovascular disorders, dementia, and depression.
Previously this immune system dysfunction was studied in peripheral systems (organs other than the brain). The study published in Nature showed this process in the brain and highlighted it’s different than anything studied before.
OUR BRAIN: WHAT IS DENTATE GYRUS AND WHY IS IT IMPORTANT?
Before proceeding, let’s understand what “dentate gyrus” is and why is it important.
Our brain has sections. Behind our ears, we have the ‘temporal lobe” of the brain. It is the second-largest lobe of our brain. “Hippocampus” is a complex structure. It is embedded here and it mediates many higher brain functions such as memory and learning.
“Dentate gyrus” is the part in the hippocampus where all the sensory signals (like sight, touch, etc) merge and help in forming a memory of these triggering signals. It plays an important role in memory and learning.
Our body immediately reflects any problem in the brain. Damage to the dentate gyrus results in cognitive issues, namely difficulty in learning in selected tasks.
We discussed the brain because, with age, people start experiencing problems related to memory-weakening or total loss. Furthermore, some aged elders cannot process information the same way as young people. They become indecisive, are more sensitive to temperature changes, and have an altered attention span.
Scientists discovered something interesting through their experiments on the immune system and age. The criminal behind “age-related symptoms” was none other than the system responsible for protecting us. Our immune system.
CELLS OF IMMUNE SYSTEM KILLS BRAIN CELLS
So let us try to understand how our immune system turns against us with time. Our defense cells are activated whenever there is a dangerous substance in the body. Namely, these include neutrophils, the famous B and T cells, macrophages, and also natural killer cells.
The natural killer cells do as the name suggests, they find their target cell and kill it-no questions asked.
When we get old, our brain cells in the dentate gyrus, called “neuroblasts” become senescent. As a result, they do not divide into further neurons. When they get are arrested in time (due to nerve aging), they start secreting some chemicals called “cytokines”.
Cytokines are the messengers of the immune system. Now, The brain releases Cytokines, IL-27 in particular, and attracts the natural killer cells to the brain.
IMMUNE CELLS IN THE BRAIN:
Once the natural killer cells reach the brain, they find themselves a feast-the aged neuroblast- and start killing them. The neuroblasts cannot form new neurons. As result, we start losing brain cells, quite literally.
A team of Chinese researchers, Wei Na Jin and team conducted this experiment on mice. They wanted to see if this natural killer cells accumulated in the other organs of the body too or not.
They concluded that this only happened in the brain. Brains of aged mice had raised levels of natural killer cells. This contributed to symptoms of old age. The natural killer values coincided with those found in the brain of aged human subjects.
CAN HUMANS LIVE LONGER?
The question is that what does this information mean for us as humans.
Scientists identified how the immune system and age were related. They showed through experiments that the brain’s aging and immune system could together cause mental problems. Now they need ways to control the overactive brain and the cells in the brain that caused immune system dysfunction. By finding a way to control the neuroblast senescence and halting the process of losing brain cells, researchers might be able to crack the code of prolonging life.
Researchers can now search for a strategy to alleviate the harm done by the immune system and age. So, if these natural killer cells can eliminate the abnormal brain cells, then normal brain functions can be retained. This can prolong the average years of life.
This study provided a possibility, a sliver of hope that in near future they dream of prolonged life that can become a reality.
