How does your immune system age, and what does that mean for how long you stay healthy?
Your immune system ages in measurable ways: it responds less effectively to new threats and maintains a smouldering inflammation that is associated with cardiovascular disease, dementia and cancer. An active lifestyle is the best-supported way to slow that process.
The immune system loses two things at once as the years pass: firepower and calm. The firepower erodes because the number of fresh, unused immune cells declines. Those cells are needed to recognise and tackle new threats, such as an unfamiliar virus or a new vaccine. At the same time, aged immune cells accumulate. They do their job less well but remain present, and they secrete substances that keep the body in a state of permanent, low-level inflammation.
That smouldering inflammation is called inflammaging and is probably the most dangerous aspect of immune ageing. People with higher levels of inflammatory substances in their blood face a greater risk of cardiovascular disease, dementia and cancer. Whether the inflammation actually causes these diseases or merely correlates with them has not yet been conclusively established, but the association has been demonstrated repeatedly. Aged immune cells also sustain the inflammation through a self-reinforcing downward spiral: the more of those cells, the more inflammatory substances, the more damage.
One direct practical consequence is that vaccines work less well in older people. This is one of the most thoroughly documented effects of immune ageing and explains why influenza or pneumococcal disease can hit older people so hard, even when they have been vaccinated. The same pattern played a role in the severe COVID-19 course seen in older individuals.
What is interesting is that not all immune change with age turns out to be harmful. In people who reach a hundred years of age, researchers observe that the immune system has reorganised itself in its own way, in a way that is associated with a long and relatively healthy life. Whether inflammaging is therefore purely damaging, or also represents an adaptive strategy, remains an open question.
Diet and exercise appear to be able to dampen the smouldering inflammation. The evidence for this is not yet strong enough for firm recommendations, but researchers view a lifelong healthy lifestyle as the most promising and safest approach. Changes in the gut microbiome also play a role in immune ageing: a less diverse gut microbiome in older age is associated with more inflammation, although it is not yet certain whether the gut microbiome is the cause or a consequence.
This answer rests on eight review studies, including two systematic reviews. Those broad literature overviews are genuinely informative, but the authors themselves point to a major limitation: the studies examining specific immune markers (such as changes in T cells) differed so greatly in measurement methods and reporting, and the risk of bias was so high, that the two systematic reviews were unable to calculate reliable summary figures. Concretely: a quantitative summary of cell markers, inflammatory substances and telomere length was impossible due to inconsistent protocols and incompletely reported measurements. The broad patterns (inflammation rises, vaccine response declines) are consistent and well described; the finer details about which specific cell characteristics are the best indicator are far less certain.