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similar for the innate arm of the immune system,
especially among people living at latitudes where there
isn’t enough winter sunlight for their skin to synthesise
the molecule. A 2017 review of the evidence for taking
vitamin D supplements concluded that it helps prevent
upper respiratory tract infections. About 1000 to 2000
IUs should be safe and beneficial, says Wu, but people
shouldn’t go higher than that because, once more, big
doses actually suppress T-cell function.
But maybe popping pills isn’t the best approach.
Exercise is a proven immune-boosting strategy. It keeps
the thymus youthful, the heart-shaped patch of
lymphatic tissue, located beneath your breastbone,
where new T-cells mature before being released on
active duty. T-cells are pivotal in the adaptive immune
response, the more targeted part of the system.
The thymus degenerates with age, and our T-cell
count with it, with consequences for our ability to fend
off new pathogens. However, when Janet Lord at the
University of Birmingham, UK, and her colleagues took
blood samples from 125 amateur cyclists aged between
55 and 79, many of whom had been regularly riding
long distances for decades, they found that their T-cell
counts were similar to those of much younger people
and their thymuses were youthful.
Exercise is also good for our neutrophils, a crucial
first line of defence against bacterial invaders. When
bacteria are detected, neutrophils squeeze out of the
blood vessel and barrel towards their target, engulfing
them like Pac-Man, spraying them with deadly
chemicals, or disgorging their DNA and throwing it
around the invaders like a net.
The ability of neutrophils to tunnel through tissues
and find their target becomes increasingly erratic as we
age. Older neutrophils can still detect invaders, but
often blunder haphazardly through tissue or charge off
in the wrong direction. Not only does this reduce the
speed and efficiency of the immune response, it can
also result in friendly fire – a leading cause of the
generalised low-grade inflammation that creeps
throughout our bodies as we age.
In a separate study, Lord and her colleagues
measured exercise levels and neutrophil migration in
211 older adults, and found that those doing 10,000
steps a day on average had neutrophils as good as those
of young adults. Neutrophils don’t kill viruses, so they
won’t prevent you from catching diseases like covid-19
or influenza, but they will shore up your defences
against bacteria – including helping to protect against
secondary infections, such as bacterial pneumonia. ❚