With age, the human immune system becomes less effective at tackling infections and less responsive to vaccinations. At the same time, the aging immune system is associated with chronic inflammation, which increases the risk of almost all conditions linked to old age.
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The good news is that exercising and adopting the right diet may help a person maintain healthy immunity into older age.
Chimpanzees and gorillas, our closest primate relatives, live for only 10–15 years in the wild once they have reached maturity. After the human evolutionary lineage split from theirs, our ancestors’ life expectancy doubled over the next 5 million years.
Scientists believe that it remained relatively stable into the 18th century. In the 250 years between then and now, however, life expectancy more than doubled again due to improvements in sanitation and healthcare.
We live in a time of high average life expectancies. However, our long evolutionary history has adapted us for different lifestyles (and even life expectancies), and these have changed drastically.
As a result, immunity not only weakens in older age; it also becomes imbalanced. This affects the two branches of the immune system — “innate” immunity and “adaptive” immunity — in a double whammy of “immunosenescence.”
“Innate” immunity, which is our first line of defense against infections, fails to resolve after the initial threat has passed, causing chronic, systemic inflammation.
“Adaptive” immunity, which is responsible for remembering and attacking particular pathogens, steadily loses its ability to defend against viruses, bacteria, and fungi.
Chronic, low-grade inflammation is associated with almost all conditions linked to older age, including type 2 diabetes, cardiovascular disease, cancer, and dementia. It also plays a leading role in certain autoimmune conditions that are more common in older adults, such as rheumatoid arthritis.
Meanwhile, the loss of adaptive immunity that comes with older age not only makes people more susceptible to infections; it can also reactivate dormant pathogens that were previously suppressed.
In addition, the weaker adaptive immunity of older adults means that their bodies respond less strongly to vaccinations, such as the annual flu shot.
Researchers have dubbed the persistent, low-level inflammation that is implicated in almost all conditions associated with older age as “inflammaging.”
The authors of a study in the journal Frontiers in Immunology explain:
“While inflammation is part of the normal repair response for healing, and essential in keeping us safe from bacterial and viral infections and noxious environmental agents, not all inflammation is good. When inflammation becomes prolonged and persists, it can become damaging and destructive.”
After an initial infection or injury, younger people’s immune systems switch to an anti-inflammatory response. This does not appear to happen as effectively in older adults. This is due to the accumulation of aged, or “senescent,” immune cells.
Senescent cells have shorter telomeres, which are the protective caps at the tips of chromosomes. Just as the plastic caps on the ends of shoelaces prevent them from fraying, telomeres prevent vital genetic material from becoming lost when the chromosome is copied during cell replication.
Telomeres get a little shorter every time a cell divides, until, eventually, division has to stop completely. If the cell survives, it becomes steadily more dysfunctional.
Senescent immune cells produce more immune signaling molecules called cytokines, which promote inflammation. Specifically, they churn out more interleukin 6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha).