How Temperature Changes Affect the Immune System

When the temperature rises, colds and the flu are less common. Do you know how temperature changes affect the immune system?
How Temperature Changes Affect the Immune System

Last update: 10 October, 2021

Summer is right around the corner, and the flu and other viral processes typical of winter are now a thing of the past. The increase in temperature in Western countries has always been related to a decrease in infectious diseases.

Mucus and colds are typical of winter, while congestion and allergies are typical of spring. However, these conditions don’t seem to affect people as much during the summer. Do you know how the immune system reacts to temperature changes?

We’ll tell you about it below.

Fewer viruses and more bacteria, depending on the temperature

It’s essential to note that, regardless of the effect of heat or cold on the immune system, the dynamics of germs and pathogens play an important role in the infectious cycles of the population:

  • For example, viruses such as colds or the flu spread more effectively in cold and dry environments, since their three-dimensional structure doesn’t withstand high temperatures well and humidity seems to favor their early precipitation. Therefore, they’re more common during the winter.
  • In contrast, various pathogenic bacteria’s ability to replicate increases at high temperatures. This is the case of the genus Salmonella, for example. This is why its incidence increases during the summer.
  • Zoonotic diseases spread by insects also increase at high temperatures, as they tend to proliferate in hot climates. For example, mosquitoes and Zika fever, malaria, and others.
The Aedes aegypti mosquito.

This article may also interest you: Why the Flu Spreads More in the Winter

How temperature changes affect the immune system

To answer this complex question, we chose a recent study the University of Tokyo conducted on mice. Although these results can be studied to predict human dynamics, they shouldn’t be considered absolute realities but biological approximations instead.

The scientists housed various sample mice groups under different temperatures: 39.2°F, 71.6°F, and 96.8°F. The scientists infected them with the flu virus.

The immune system of the mice in very hot temperatures didn’t respond effectively. It’s also worth noting that the mice in very high temperatures lost 10% of their body mass in the first 24 hours.

This poses different questions: is the decrease in the effectiveness of the immune system due to poorer nutrition and possible hydric stress or does heat change the expression patterns of the body’s genes?

There’s no clear answer yet, meaning that experts need to conduct more experiments to elucidate the dynamics between viral infections and temperature changes.

For more information, read this article: Immunonutrition: Can We Stimulate the Immune System?

Beyond the flu

But immune system fluctuations go far beyond what we’ve discussed so far. The activity of a quarter of human genes varies according to the season of the year, as some are more active during the winter, while others are during the summer.

A study by the director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory (DIL) evaluated how seasonality affects molecular expression in humans.

The scientists analyzed people from the United Kingdom, the United States, Australia, and the Gambia. They concluded that they all had different variation patterns. For example, they observed that there was a trend towards pro-inflammatory gene expression during the winter in Europe. During this season of the year, the immune system is more prone to inflammatory responses.

This raises more questions than answers since it’s suspected that the gene expression that modifies the immune response varies according to temperature and hours of sunlight. However, experts need to do a lot more research to understand the specific mechanisms.

A forest in the summer and winter.

Seasonality, temperature changes, and changes in habits

Thousands of factors condition humans’ bodily response to pathogens. Therefore, it’s very difficult to quantify the effectiveness of the immune system in the different seasons of the year. For example, during the summer, the incidence of gastrointestinal diseases increases.

But why does this happen?

There are several possible causes. Although they’re all are possible explanations, experts have yet to confirm them:

  • Increased temperatures promote bacterial growth in food.
  • Humans travel more during the summer and are exposed to bacteria that they aren’t used to, which makes them prone to suffering from diarrhea.
  • The immune system fails more at high temperatures.
  • Eventual dehydration and weight loss during the summer decrease the body’s immune response.

The immune system is influenced by numerous factors

As you can see, we have to take many different factors into account regarding this issue. Likely, there isn’t a definite answer.

Science needs to keep putting in the work to clarify the possible answers to these questions. For this purpose, experts need to conduct more studies to clarify these concepts.

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  • Moriyama, M., & Ichinohe, T. (2019). High ambient temperature dampens adaptive immune responses to influenza A virus infection. Proceedings of the National Academy of Sciences116(8), 3118-3125.
  • Dopico, X. C., Evangelou, M., Ferreira, R. C., Guo, H., Pekalski, M. L., Smyth, D. J., … & Prentice, A. M. (2015). Widespread seasonal gene expression reveals annual differences in human immunity and physiology. Nature communications6(1), 1-13.
  • ¿Cómo afectan las estaciones al sistema inmunitario? Eluniverso. Recogido a 1 de junio en https://www.eluniverso.com/vida-estilo/2015/05/12/nota/4866086/como-estaciones-afecta-sistema-inmunitario.