How Aging Affects Your Immune System

Most illnesses disproportionately affect adults ages 65 and older. This scientific fact became all too stark during the peak of the COVID-19 pandemic. Those in their 20s had little risk of severe infection compared to people over 80. And every flu season we get the same sobering reminder, with people over 65 being at far higher risk than young adults.

It might not make sense at first. If previous exposure to bacteria and viruses strengthens the immune system, shouldn’t older people have the best defenses?

To help guide us through this subject, we talked to world-renowned National Jewish Health immunologist Philippa Marrack, PhD, whose discoveries have been instrumental for understanding how the body reacts to disease.

At the Cellular Level

Whether you’re an orca or an amoeba, to live is to be under attack. So every living thing has developed a defense system. In humans, two of the central tools of this system are neutrophils and T cells.

Neutrophils are like buffet diners. They’re not very picky, just hungry, gobbling up anything they recognize as foreign through chemical signals. And they’re essential to our survival. “Actually, if you don't have neutrophils, you die in about a day,” said Dr. Marrack, “because every bacterium in the world can get inside you and start doing damage.”

Dr. Marrack explained that neutrophils help form what we call our innate immunity. As soon as we’re born, our neutrophils are ready to go, and hungry for any kind of bacteria or viruses they can find.

T cells, on the other hand, are the immune system’s specialists. Compared to neutrophils, they’re like fine dining connoisseurs. And they all have an acquired taste for very specific items. This is because T cells have receptors that can only bind to unique proteins. To give you an idea of just how unique, T cells have about 10,000 of these receptors on their surface, and the configurations are different from cell to cell.

Each of these T cells is waiting for something very special to come along that it can bind with, and there are so many different variations of T-cell receptor configurations. Even if something comes along that the immune system has never encountered before (like a certain coronavirus), chances are there’s a T cell in your body to match.

“When T cells bind with a virus or bacterium, that binding sends a signal to the cell,” explained Dr. Marrack. “And that signal says, ‘Divide like crazy!’ So if you start off with just one, you have about a million of them six days later, and their job is to kill the coronavirus or whatever they’ve found.”

This is what we call acquired immunity. Even after an infection has been dealt with, some of those T cells that bonded with the specific invader will stick around, just in case you come in contact with it again.

What Happens When We Get Older?

Now that we understand the basics of innate and acquired immunity, we can return to the question of age. For T cells, those specialists that are essential for building acquired immunity, division is crucial. But as we get older, our T cells divide at a much slower rate. For instance, for a child under 10, T cells will divide once every 8-12 hours. For someone in their late 70s though, that rate will slow to about once every 16 hours.

This also happens to other immune cells in the body, such as neutrophils and B cells, which produce antibodies that help combat bacteria.

Because the immune system depends on T cells and other tools dividing at a rapid rate, every hour makes a big difference. “A younger person with an infection will soon have an army of T cells to combat it,” said Dr. Marrack. “An older person will have much less, and it will take longer to gather those resources, which means the infection has a better chance of winning. For instance, seniors often die of pneumonia. And pneumonia is caused by the buildup of particular bacteria, usually strep in your lungs.”

Also, after you’ve successfully mounted a defense against a virus or bacteria in the past, a team of T cells specialized for that infection will stick around. As this happens again and again over time, Dr. Marrack explains, it effectively “squeezes out” T cells that could attack different infections. This leaves you less prepared for new infections. It’s similar to memory and learning. For both your mind and body, the older you get, the harder it is to teach yourself new tricks.

Strengthening Your Immune System

Despite what you may have seen in advertisements, there’s no way to reverse the aging process. If we could all maintain the immune systems of our 16-year-old selves, we’d live all live much longer lives.

As Dr. Marrack pointed out, we are living longer lives compared to our ancestors. Modern antibiotics and vaccines do a lot to assist our immune systems. So, making sure you’re taking your doctor’s advice and keeping up to date on your medication can compensate for a lot.

And even though exercise and a healthy diet won’t exactly turn back the clock, they’ll ensure that your immune system is at its best the next time another bug comes around.