The immune system is challenged during flu season

It's that time of year again: it's getting colder outside, the days are getting shorter, and inside you're hearing more and more people cough or sniffle. Before you know it, a box of tissues will be open for you too. Flu season has begun.

Yet, it's not just bad luck if you get sick. Your immune system is a complex interplay of various factors. Your immune system reacts to stimuli from your environment every time and tries to find a balance in it. If that balance is disturbed, for example by stress, lack of sleep, or unhealthy nutrition, you are more susceptible to pathogens that you would normally easily ward off. In this blog, you will read what the immune system is, why you are more susceptible to illnesses during winter, which factors influence the immune system, and what practical tips there are to help yourself.

What is the immune system?

Your immune system is your body's defense mechanism. It protects you against pathogens such as viruses, bacteria, and fungi, as well as against cells that no longer function properly. Your body can do this through a network of various organs, cells, and signaling substances that constantly communicate with each other.

The first layer of the defense mechanism is on the outside. These are your skin and mucous membranes. They form a physical barrier that prevents intruders from entering. If something still slips through, the second layer becomes active. These are your white blood cells, lymph nodes, and spleen. They can recognize pathogens and eliminate them. Meanwhile, the immune system learns from these pathogens, so they can be eliminated more quickly and effectively next time.

A well-functioning immune system is not constantly active. It reacts when necessary but then calms down again. The balance between action and relaxation is very important for your health, especially during times when your immune system is being challenged.

Why are people more susceptible during flu season?

In winter, it seems like everyone around you gets sick, but that's not because it's colder. Various biological and environmental factors play a role that collectively make your immune system work harder during this period.

Firstly, in winter, the amount of sunlight we receive decreases, which reduces the production of vitamin D. This fat-soluble hormone plays an important role in activating your immune system. A deficiency can cause you to react more slowly to pathogens (Prietl et al., 2013).

We also spend more time indoors in cold weather, often in poorly ventilated spaces where viruses spread more easily. The air in heated rooms is also drier, which dries out the mucous membranes in your nose and throat. These mucous membranes provide important protection against pathogens, but if they are drier, viruses can attach more easily.

In addition, lifestyle plays a role. During the darker months, we often exercise less, and our biorhythm is often somewhat disturbed. Stress, holidays, and alcohol consumption can exacerbate this. All these factors together ensure that your immune system temporarily works less efficiently, and this happens precisely when pathogens are most active.

FACTORS INFLUENCING THE IMMUNE SYSTEM            

Your immune system continuously reacts to signals from the environment and to the choices you make daily. Some factors strengthen your immune system, while others weaken it. It's all about the balance between exertion and recovery, tension and relaxation, nutrition and rest.

Nutrition

An important building block of the immune system is nutrition. Immune cells need sufficient proteins, fatty acids, vitamins, and minerals to function properly. Especially vitamins B, C, D, and A, zinc, and selenium play a role in the production and activation of defense cells (Munteanu & Schwartz, 2022). You can mainly get these from animal foods, such as grass-fed beef. Gut health is also very important. A large part of the immune system is located in the intestinal wall. Here, good bacteria help to regulate inflammatory reactions (Purchiaroni et al., 2013). In addition, alcohol causes low-grade inflammation, which weakens your immune system.

Sleep

Additionally, sleep has a direct impact on the immune system. During deep sleep, recovery processes are activated, and cytokines (signaling substances that immune cells use to communicate with each other) are produced, which are necessary to fight infections. Poor sleep can disrupt this regulation (Besedovsky et al., 2012; Singh et al., 2025). 

Biorhythm

Your immune system does not work independently of your biological clock. All processes in your body follow a biorhythm of approximately 24 hours. This also applies to your immune cells: some are mainly active during the day, others at night. When your biorhythm is disturbed, for example by irregular working hours, stress, or too little daylight, your internal clock also becomes deregulated. This can cause immune cells to be active at the wrong time and respond less effectively to pathogens. Regular bedtimes and exposure to daylight help keep your biorhythm stable and thus also support your immune system (Shephard & Shek, 1997).

Stress

During prolonged stress, cortisol is continuously released. This hormone is positive in the short term, but with persistently high cortisol levels, your immune system comes under pressure (Pruett, 2003).

Exercise

Regular exercise stimulates the circulation of immune cells and reduces inflammatory responses. It is important to avoid excessive exertion as much as possible, as this can actually fuel inflammatory responses (Simpson et al, 2020).

The immune system is therefore not an isolated system, but a reflection of your overall health. The better you ensure rest, regularity, and recovery, the better your body is able to stay in balance.

PRACTICAL TIPS

• Focus on protein-rich nutrition. 
• Invest in good sleep hygiene.
• Get daylight at different times of the day.
• Avoid stress where possible by planning sufficient moments of relaxation throughout the day.
• Exercise regularly, but don't overdo it.

Book Recommendations

• "The Immunity Fix" by Dr. Jenna Macciochi
• "Optimal Immunity" by Dr. Rudy Proesmans

References

• Besedovsky, L., Lange, T., & Born, J. (2012). Sleep and immune function. Pflügers Archiv-European Journal of Physiology, 463(1), 121-137.

• Munteanu, C., & Schwartz, B. (2022). The relationship between nutrition and the immune system. Frontiers in nutrition, 9, 1082500.

• Prietl, B., Treiber, G., Pieber, T. R., & Amrein, K. (2013). Vitamin D and immune function. Nutrients, 5(7), 2502-2521.

• Pruett, S. B. (2003). Stress and the immune system. Pathophysiology, 9(3), 133-153.

• Purchiaroni, F., Tortora, A., Gabrielli, M., Bertucci, F., Gigante, G., Ianiro, G., … & Gasbarrini, A. (2013). The role of intestinal microbiota and the immune system. European Review for Medical & Pharmacological Sciences, 17(3).

• Shephard, R. J., & Shek, P. N. (1997). Interactions between sleep, other body rhythms, immune responses, and exercise. Canadian journal of applied physiology, 22(2), 95-116.

• Simpson, R. J., Campbell, J. P., Gleeson, M., Krüger, K., Nieman, D. C., Pyne, D. B., … & Walsh, N. P. (2020). Can exercise affect immune function to increase susceptibility to infection?. Exercise immunology review, 26, 8-22.

• Singh, K. K., Ghosh, S., Bhola, A., Verma, P., Amist, A. D., Sharma, H., … & Sinha, J. K. (2025). Sleep and immune system crosstalk: implications for inflammatory homeostasis and disease pathogenesis. Annals of Neurosciences, 32(3), 196-206.