One thing that the COVID-19 pandemic has made clear is that some people who get the virus don’t suffer much, while others become very sick indeed. And while the elderly have been particularly hard hit, some do survive—even centenarians. As for younger people, whose immune systems are supposed to be more robust, many have nevertheless died of it. So, what factors give some people a stronger immune system than others, regardless of age? What does it mean if, for example, your partner or child gets sick, and you don’t—or vice versa?
We know that our immune-system function slowly declines with age. Just like when you see a photo of yourself from ten years ago versus one taken this afternoon, you see changes in your face, skin, and hair colour. It takes time. “Same process with your immune system,” says Dr Insoo Kang, associate professor of medicine and director of allergy and immunology at the Yale School of Medicine.
Kang has been studying human ageing for 20 years. “Immune cells, especially CD8+ T cells [a type of white blood cell], change with ageing. We see fewer of those CD8+ T cells, which are needed to recognise newly emerging microorganisms like the COVID-19 virus. It happens to everyone on some level, just not at the same rate.”
The difference in rate of decline between individuals is one of science’s big mysteries. The immune system is complex, but most of us understand the basics: our body detects an intruder—a virus, bacterium, parasite or foreign object—and produces white blood cells to combat the problem. How many of these cells you produce when you’re, say, 73 versus when you were 45 is the million-pound question. And it’s at the heart of the COVID-19 pandemic as well: how does a 104-year-old man survive when people half his age, or younger, do not?
Fortunately, we won’t always be in a pandemic. But we can use this one to gain an understanding of how our immune system changes with age— and how it might be possible to slow the decline and raise our immune reserves for the next time we get sick.
Immunity And Ageing
Our body simply doesn’t produce as many immune cells as we get older, says Dr Atul Butte, distinguished professor of epidemiology, biostatistics, and pediatrics at the University of California, San Francisco. “And no one really knows why.”
Butte worked with a research team on an extensive review of 242 immunity studies that revealed patterns in how our immune systems change as we get older. Certain key immune cells—B cells (white blood cells that make antibodies to fight infection) and T cells (white blood cells that attack viruses)—become fewer in number with age. For example, we possess two broad categories of T cells: “memory” cells that have encountered a certain pathogen and “remember” how to fight it; and “naive” cells, like the CD8+ type mentioned earlier, that have yet to fight anything. “We’ve seen especially that the number of naive T cells seems to be lower as we age,” Butte says.
So let’s say COVID-19 shows up. Nothing we’ve seen before as humans matches this one, so we have no memory T cells to mobilise (although new research suggests some of us may have formed some defences if we’ve had brushes with previous coronaviruses). The naive cells have to take on the fight, and older folks have fewer of those to fight with. That makes most of these people more vulnerable, but not all, because not everyone’s immune system declines in the same way. For example, another factor Butte observed in his study review: some healthy older people had little or no decline in T cells. Some had as many as younger people, and women seemed to have higher amounts in general as they aged.
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