Adrian Liston at the Catholic University of Leuven in Belgium and his colleagues have looked at blood samples from 670 healthy people aged between 2 and 86. For every person, the team counted the amounts of immune cells in each of 54 different activation states. The group then analysed further samples from a quarter of these volunteers over a six-month period.
The immune profiles tended to be remarkably stable, says the team, and even when disturbed, they returned to a similar baseline state. When people caught gastroenteritis, or had a flu vaccine, their immune systems were only temporarily perturbed before returning to normal.
This is reassuring, says Valerie O’Donnell at Cardiff University, UK. “The immune system deals with the stressor and then recovers well.”
In the family way
But Liston spotted an interesting pattern – the immune systems of people who live together seem to be remarkably similar. “There is a 50 per cent reduction in variation, which is an extremely profound effect,” says Liston. “It’s a larger effect than you see over 40 years of ageing.”
People in relationships tend to adopt similar diets and lifestyles, a phenomenon known as spousal concordance. The partner of a teetotal person is likely to drink less, for example. It is probable that some aspect of a couple’s shared environment is the cause of their converging immune systems, says Liston.
There’s evidence for a few potential culprits. Exposure to pollution can affect the immune system, and two people living together are likely to experience similar exposures. Holden Maecker of Stanford School of Medicine in California thinks the immune systems of cohabiting couples could be shaped by sharing common viruses with each other.
Microbial conduits
People who live together also seem to have similar collections of gut bacteria, which are important for immune functioning. This similarity could be influenced by the microbes found on household surfaces or in dust, or perhaps because of children. All the couples included in the study were parents, so their children could have acted as microbial conduits, says Liston.
“It’s not particularly nice to imagine, but the easiest way to transmit gut bacteria is the faeco-oral route – and parents could both be changing a baby’s nappy,” Liston says.
Children also pick up bacteria from their mother during birth and breastfeeding, and this may be passed to their father when he kisses his baby.
It is hard to know whether this immune convergence might be good or bad for your health, says O’Donnell. “It could go either way depending on how they converge,” she says.
Some changes might make a person more susceptible to infection, but they could also strengthen their defences against others. “We don’t really have a way to tell if someone has a ‘good’ immune system,” says Maecker.
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