The immune system is challenged during flu season

It's that time of year again: the weather is getting colder, the days are getting shorter, and inside you hear coughing or sniffling more and more often. Before you know it, you'll be opening a box of tissues. Flu season has begun.

However, getting sick isn't all bad luck. Your immune system is a complex interplay of various factors. It constantly responds to environmental stimuli and tries to find a balance. If that balance is disrupted, for example, by stress, lack of sleep, or an unhealthy diet, you're more susceptible to pathogens you would normally easily ward off. In this blog post, you'll read about what the immune system is, why you're more susceptible to illness during the winter, which factors influence it, and some practical tips 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 cells that no longer function properly. Your body can do this through a network of various organs, cells, and signaling molecules that constantly communicate with each other.

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

A well-functioning immune system isn't constantly active. It responds when needed, but then calms down again. The balance between activity and relaxation is crucial 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 is getting sick, but that's not because it's colder. Several biological and environmental factors contribute to this, putting your immune system under increased strain during this time.

First, in winter, the amount of sunlight we receive decreases, which reduces vitamin D production. This fat-soluble hormone plays a key role in activating your immune system. A deficiency can slow your response 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 when they are drier, viruses can more easily attach themselves.

Lifestyle also plays a role. During the darker months, we often move less, and our biorhythms are often somewhat disrupted. Stress, holidays, and alcohol consumption can exacerbate this. All these factors together cause your immune system to temporarily function less efficiently, precisely when pathogens are at their most active.

FACTORS AFFECTING THE IMMUNE SYSTEM

Your immune system is constantly responding to environmental signals and the choices you make every day. Some factors strengthen your immune system, while others weaken it. It's all about the balance between stress and recovery, tension and relaxation, nutrition and rest.

Power supply

Nutrition is a key building block of the immune system. Immune cells require sufficient protein, fatty acids, vitamins, and minerals to function properly. Vitamins B, C, D, and A, zinc, and selenium, in particular, play a role in the production and activation of immune cells (Munteanu & Schwartz, 2022). These can primarily be obtained from animal foods, such as grass-fed beef. Gut health is also crucial. A large part of the immune system is located in the intestinal wall, where beneficial bacteria help regulate inflammatory responses (Purchiaroni et al., 2013). Furthermore, alcohol causes low-grade inflammation, which weakens your immune system.

Sleep

In addition, sleep directly influences the immune system. During deep sleep, restorative processes are activated and cytokines (signal molecules through which immune cells 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 operates in conjunction with your biological clock . All processes in your body follow a circadian rhythm of approximately 24 hours. This also applies to your immune cells: some are primarily active during the day, others at night. When your circadian rhythm is disrupted, for example, by irregular work hours, stress, or insufficient daylight, your internal clock is also disrupted. This can cause immune cells to be active at the wrong times and less responsive to pathogens. Regular bedtimes and exposure to daylight help keep your circadian rhythm stable and thus also support your immune system (Shephard & Shek, 1997).

Stress

During prolonged stress, cortisol is released repeatedly. This hormone has a positive effect in the short term, but persistently high cortisol levels actually put your immune system under pressure (Pruett, 2003).

Movement

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

The immune system isn't a separate system, but a reflection of your overall health. The better you ensure rest, regularity, and recovery, the better your body is able to stay balanced.

PRACTICAL TIPS

• Focus on protein-rich foods.
• Invest in good sleep hygiene .
• Grab daylight at different times.
• Avoid stress where possible by planning sufficient relaxation time during the day.
• Exercise regularly, but don't overdo it.

Book tips

• “The Power of Immunity” 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, RJ, Campbell, JP, Gleeson, M., Krüger, K., Nieman, DC, Pyne, DB, … & Walsh, NP (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.