JUST as many parts of our body change as we get older, so does our immune system. But contrary to popular belief, emerging evidence suggests it isn’t all downhill.
Let’s start at the beginning. Because of their lack of previous encounters with pathogens, young children are vulnerable to all kinds of infections. Newborn babies have some protection thanks to antibodies that cross the placenta to reach the fetus during pregnancy and linger in their bodies for weeks to months. Some antibodies are also passed on through breast milk.
Vaccines are recommended for babies in the order in which protection from these antibodies wanes. For instance, a vaccine against whooping cough is given at 8 weeks of age, because so-called maternal antibodies to this infection fall quickly.
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Once babies and children start mixing with other children at nurseries and schools, there is a surge in respiratory illnesses, stomach bugs and other infections they haven’t encountered before. We gradually encounter more and more pathogens, leading to immunity against them that can last a lifetime. Even infections that are too mild to notice can lead to immunity.
In adulthood, pregnancy leads to temporary changes in the immune system, with some immune cells becoming less active to make sure the fetus isn’t rejected. This can mean pregnancy leads to a higher risk of infections, such as flu and covid-19. However, early work suggests that part of the immune system, called . In lab tests, NK cells from those who are pregnant are more active against cancer cells than NK cells from those who aren’t.
This article is part of a special series on immunity, in which we explore:
As people get older, the immune system generally offers less protection. There is , an organ in the neck where T-cells are trained to attack pathogens. B-cells, which make antibodies, . And after menopause, lower levels of the hormone oestrogen may also contribute to weaker immune activity.
Immunity declines
The immune system also wastes resources targeting a pathogen called cytomegalovirus (CMV). Most people have been infected by this virus without noticing, but it persists in tissues such as the salivary glands. As we age, an increasing proportion of T-cells are dedicated to attacking this virus. “CMV is constantly stimulating the immune response, but you can’t get rid of it,” says at University College London.
The general immune decline with ageing contributes to older people being worse hit by infections such as covid-19, flu and pneumonia. They respond less well to vaccines too. This decline can also result in the reactivation of viruses previously held in check, such as the chickenpox virus, leading to shingles. And it may contribute to the increasing incidence of cancer as people age.
But older people aren’t always worse off. In 2009, a new strain of flu crossed from pigs to humans, which led to a swine flu pandemic. In contrast to most flu strains, people over 65 were less likely to become sick or die from swine flu than younger people, which was probably because they had some immunity from related flu strains that had circulated in their childhood.
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