How the immune system changes with age and why

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.

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, …