Does the immune system really change with the seasons

DEAR DOCTOR K:

A friend heard about a study that said a person’s immune system changes with the seasons. That seems incredible to me. But if it’s true, it’s fascinating. Do you know what she is talking about?

DEAR READER:

I think I know the study she is referring to. Before describing what it found, it’s worth talking a bit about the immune system and also about genes.

First of all, that the immune system should change in response to any external circumstance — like the weather, or the season — should not be surprising. The immune system is constantly changing as it responds to a changing set of foreign threats. Different germs, different non-infectious foreign substances (such as pollen, animal hair, dust) are entering our bodies all the time. A healthy immune system is always refocusing its attack as it encounters new foreign substances.

The immune system is like an army; it has many different types of cells that have different functions. Think of each group of cells as a platoon that does a particular job. It also has a hierarchy, with the cells at the “top” directing the actions of all the other cells — just like the generals and admirals in the armed forces. Those top cells send orders in the form of immune system chemicals.

For the cells of the immune system to function properly, the right genes in those cells need to be turned on (and other genes turned off) at the right time. In the past 30 years, research scientists have developed ways of determining which genes in a cell (or a related group of cells) are turned on and off.

So much for the biology lesson, and on to the study I think your friend was talking about. It was conducted by a scientific group in Europe that obtained blood samples from thousands of people living in both the Northern and the Southern hemispheres. Some people lived well north (or south) of the equator, where the seasons are most dramatic. Others lived near the equator all year, where the seasons are the least dramatic.

In the blood samples, the researchers measured which immune system genes were turned on. They found that some immune system genes were turned on pretty much all year, without any seasonal variation. But in both hemispheres, some immune system genes were turned on quite differently during summer than during winter.

The study also found, not surprisingly, that in parts of the world where infectious diseases are most common during certain times of the year, many more immune system genes were turned on during those times. For example, genes that turn on protective inflammation were activated more often during the rainy season in equatorial Africa — a time when malaria is rampant.

There are no practical applications that follow from this study — at least not that I can see, and not yet. But I’m glad you asked about it, because I agree with you that the study is interesting. It underlines just how dynamic our immune system is — and has to be — to do its job.