How Does Temperature Affect The Immune System?

Summer is approaching and, with it, it seems that we are leaving behind the seasonal flu and the viral processes typical of winter. The increase in temperature in Western countries has always been correlated with a decrease in infectious diseases.

Winter is the time for snot and colds, spring is for congestions and allergies, and summer seems free of pathologies. Still , do you know how the immune system reacts to variations in temperature? We will tell you about it below.

Fewer viruses and more bacteria depending on the temperature

It is essential to highlight 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 infective cycles of the population:

• For example, viruses such as colds or flu are transmitted better in cold and dry environments, since their three-dimensional structure does not withstand high temperatures well and humidity seems to favor their early precipitation. Therefore, they are predominant during winter.

• In contrast, various pathogenic bacteria increase their ability to replicate at high temperatures. This is the case of the genus Salmonella , for example, which increases its incidence during the summer.
• Zoonotic diseases transmitted by insects also increase at high temperatures, as these tend to proliferate in hot climates. Examples are mosquitoes and Zika fever, malaria and others.

How does the increase in temperature affect the immune system?

To answer this complex question, we have chosen a recent study carried out by the University of Tokyo in mice. These results can be used to predict human dynamics, but they should not be taken as absolute realities, but more as biological approximations.

The scientists housed various sample groups of mice under different temperature conditions: 4 degrees, 22 degrees and 36 degrees. They were all infected with the flu virus.

The immune system of the mice under very high temperature conditions did not respond effectively. It is also worth noting that the mice housed under high temperatures lost 10% of their body mass in the first 24 hours.

This raises different questions: is the decrease in the effectiveness of the immune system due to poorer nutrition and possible water stress, or do the expression patterns of the body’s genes change with heat?

There is no clear answer yet, but more experiments need to be done to elucidate the dynamics between viral infections and temperature.

Beyond the flu

But the fluctuations in the immune system go far beyond what has been discussed so far. The activity of a quarter of human genes varies according to the season, some being more active in winter and others in summer.

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

Individuals from populations in the UK, USA, Australia and Gambia were analyzed and all showed different patterns of variation between them. For example, in Europe a trend towards pro-inflammatory gene expression was observed in winter. During this season, the immune system is more prone to inflammatory responses.

This leaves us almost more questions than answers, since it is suspected that the gene expression that modifies the immune response varies according to temperature and hours of light. However, much research is still needed to understand the specific mechanisms.

Seasonality, temperature, and changes in habits

Human beings are conditioned by thousands of factors when it comes to responding to pathogens. Therefore, quantifying the effectiveness of the immune system in the different seasons is very complex.

For example, during the summer the incidence of gastrointestinal diseases increases. But what is this due to? We can propose various causes; all are possible explanations, but not confirmed:

• Increasing temperatures promote bacterial growth in food.
• Humans travel more and are exposed to bacteria they are not used to, making it easier to get diarrhea.
• The immune system fails more at high temperatures.
• Eventual dehydration and weight loss during the summer decrease the immune response capacity.

The immune system is influenced by numerous factors

As we can see, when evaluating this issue, many factors must be taken into account. Most likely, the answer is not found in just one of them.

However, science continues to work to clarify the possible answers to these questions. More studies are needed to clarify these concepts.