Everything you need to know about a healthy immune system

Note: This article is for general information only and does not replace medical or pharmaceutical advice. If you have any health concerns, please consult a doctor or pharmacist.

How is the immune system structured?

Our bodies are constantly confronted with viruses, bacteria and fungi. The immune system — the body's own defence system — is the central network that recognises foreign substances and responds to them.

The immune system includes organs and cells that each perform specific roles:

• Skin and mucous membranes (mouth, throat, nose, intestines): These barriers are the first physical protection. Pathogens potentially enter the body through these areas — and the first defence mechanisms also take place here.
• Tonsils: An early barrier against pathogens from inhaled air and food.
• Bone marrow: The site where immune cell precursors are produced.
• Thymus: A glandular organ above the heart where T cells mature.
• Spleen: Involved in the proliferation of lymphocytes (white blood cells), the storage of immune cells, and the removal of aged red blood cells.
• Lymph nodes and lymphatic vessels: Transport routes and collection points for immune cells and antibodies.

All of these structures work as a network — no single component acts in isolation.

The two lines of defence: innate and acquired

The immune system operates on two fundamentally different levels, which are closely interconnected.

The innate (non-specific) immune system

This system is present from birth. It responds quickly and non-specifically to a wide range of foreign substances — without needing to "know" them in advance.

It works via two types of barriers:

Mechanical barrier: Skin and mucous membranes physically prevent pathogens from entering.

Chemical barrier: Body fluids such as saliva, gastric acid, eye fluid and urine create a hostile environment for microorganisms. Cilia in the bronchi and intestinal muscles actively transport foreign substances away.

At the cellular level, two cell types are involved:

• Phagocytes: Part of the white blood cells (leukocytes). They engulf and digest pathogens. Remnants are presented to the cell surface where the acquired immune system continues the response.
• Natural killer cells (NK cells): Recognise and destroy virus-infected and tumour-altered cells without prior sensitisation.

The acquired (specific) immune system

This second line of defence develops over the course of life — through contact with pathogens. It is slower but considerably more precise than the innate system.

Early scientific insight came from the discovery of dendritic cells by Ralph Steinman in 1973. Dendritic cells act as an early warning system, alerting the acquired immune system when foreign substances enter the body. The research of Jules Hoffmann and Bruce Beutler in the 1990s was also groundbreaking, showing how the innate immune system recognises foreign substances. [1]

The acquired immune system works with two cell types:

T lymphocytes (T cells): Produced in the bone marrow, they mature in the thymus and take on various roles:

• T helper cells: coordinate the immune response
• T killer cells (cytotoxic T cells): recognise and eliminate infected cells
• Memory cells: enable a faster response when the same pathogen is encountered again

B lymphocytes (B cells): Mature in the bone marrow (hence "B cells" from "bone marrow"). Activated by T helper cells, they differentiate into plasma cells that produce antibodies (immunoglobulins) against specific pathogens.

The key difference: The innate system responds immediately but non-specifically. The acquired system requires more time on first contact, but works in a targeted and more efficient way on subsequent encounters — because it already "knows" the pathogen.

Which factors influence the immune system?

Numerous external and internal factors can influence how the immune system functions — in both directions. Science has investigated many of them in depth.

Brain and nervous system

The autonomic nervous system and the immune system are closely interconnected. The sympathetic and parasympathetic nervous systems — two nerve pathways running from the brain through the entire body — supply organs including the spleen and lymph nodes. This means nerve signals can directly influence immune cells. [4]

More recent scientific studies also suggest that certain genes influence both immunological functions and areas of the brain. A study conducted at the University of Basel identified a variant of the TROVE2 gene, which is involved in both immunological processes and memory functions. [3]

Alcohol

People who regularly consume large amounts of alcohol show altered immune responses. A study at Loyola University Chicago found that excessive alcohol consumption initially caused a strong activation of the immune system, followed by a significant drop in killer cell activity several hours later. [7] These findings are discussed in the scientific literature; for individual guidance, please consult a doctor.

Smoking

Nicotine has been shown to inhibit the formation of immunoglobulins (antibodies). [5, 6] Smokers show differences in wound healing compared to non-smokers, which current research associates with an altered immune response. The extent of this effect varies individually.

Which micronutrients contribute to the normal function of the immune system?

The European Food Safety Authority (EFSA) has reviewed the scientific evidence and authorised specific health claims for a defined set of nutrients. The following nutrients are authorised to bear the claim that they "contribute to the normal function of the immune system" — this is the official, legally recognised phrasing under EU Regulation EC 1924/2006.

Important: This EFSA authorisation refers to the normal function of the immune system as part of a balanced diet — not to the treatment or prevention of diseases. The nutrients listed do not replace medical treatment. If you suspect a nutrient deficiency, please have it assessed by a doctor.

What role does the gut play in immune function?

The gut — six to nine metres long with a total surface area of approximately 400 m² — is a central organ in the immune system. Estimates suggest that around 70–80% of all the body's immune cells are located there. [8]

The gut lining forms a physical barrier that determines which substances enter the bloodstream. The gut-associated lymphoid tissue (GALT) coordinates the local immune response.

Gut microbiome: The gut is home to trillions of microorganisms — the gut microbiome. Scientific studies show that the microbiome communicates with the immune system and is involved in regulating immune responses. A diverse gut flora is described in research as a positively associated factor.

Probiotics and prebiotics: Probiotics are live microorganisms found in fermented foods such as yoghurt, kefir and sauerkraut, as well as in food supplements. Prebiotics are indigestible fibres (e.g. inulin, oligofructose) that can promote the growth of beneficial gut bacteria. Research in this area is active; specific clinical recommendations should be discussed with a doctor.

How does sleep affect the immune system?

Sleep is an active biological process — not a passive state of rest. During sleep, particularly in deep sleep phases, important processes take place in which the immune system plays a role.

Scientific studies show that during sleep, certain immune cells are activated and messenger substances (cytokines) are produced that are involved in immune regulation. [9] Sleep deprivation is associated in research with changes in various immune parameters.

General sleep research recommendations for adults are 7–9 hours per night. Practical tips for restful sleep:

• Keep regular sleep times
• Avoid screens for one hour before bed
• Keep the bedroom cool and dark
• Reduce caffeine in the afternoon

The relationship between sleep and immune function is an area of active research. Individual sleep needs can vary.

Why does stress affect immune defence?

The autonomic nervous system and the immune system are closely linked. Chronic — meaning persistent — stress is, according to current research, a factor that can influence immune balance.

When the body experiences stress, it releases cortisol and adrenaline. Short-term stress has a mobilising effect on certain immune cells. Chronic stress, on the other hand, is associated in scientific studies with altered immune regulation — the exact mechanisms are still being researched. [4]

Measures studied in stress research:

• Regular relaxation techniques (breathing exercises, meditation, progressive muscle relaxation)
• Regular physical activity
• Social connection
• Adequate sleep

A positive social environment is described in psychoneuroimmunology — the research field studying interactions between the mind and immune system — as a factor associated with immune function.

How can diet and lifestyle support the immune system?

Diet

A balanced diet is the foundation for supplying the body with the nutrients that contribute to the normal function of the immune system. General dietary recommendations include:

• Five portions of fruit and vegetables daily (varied and colourful)
• Wholegrains as a carbohydrate source
• Regular pulses, nuts and seeds
• Oily fish one to two times per week
• Adequate fluid intake (at least 1.5 litres per day)

Why variety matters: Different coloured vegetables and fruits provide different secondary plant compounds (polyphenols, carotenoids, flavonoids), which are described in research as antioxidant. They neutralise free radicals produced during metabolic processes.

Body weight: Both significant overweight (obesity) and significant underweight are associated in research with altered immune parameters. A balanced calorie intake that meets nutritional needs is part of a healthy diet.

Hydration: Adequate fluid intake supports the function of mucous membranes — one of the first physical barriers of the immune system.

Physical activity

Moderate, regular physical activity is associated in scientific studies with various aspects of immune function. The WHO recommends at least 150 minutes of moderate activity per week. Extreme overtraining, however, can strain the body — individual adjustment is important.

Exercising outdoors combines physical activity with sun exposure, which supports the body's own vitamin D synthesis — a nutrient that, according to EFSA, contributes to the normal function of the immune system.

The acquired immune system and hardening

The acquired immune system develops through experience: contact with pathogens in childhood helps build antibodies and memory cells. Traditional hardening methods such as contrast showers, Kneipp treatments or sauna use have been practised for a long time; scientific research into their mechanisms is ongoing.

Hygiene in the right measure: Excessive avoidance of germs prevents the acquired immune system from coming into contact with pathogens it can learn from. Standard hygiene measures such as handwashing are sensible; excessive disinfection in everyday life is not necessary according to current research.

How does an allergic reaction work within the immune system?

In an allergy, the immune system reacts to actually harmless foreign substances (allergens) — such as pollen, animal dander or certain foods. The process follows a characteristic pattern:

1. First contact: The immune system identifies the allergen and incorrectly classifies it as dangerous.
2. Sensitisation: The immune system produces specific antibodies (IgE) against the allergen and stores it as "dangerous".
3. Second contact: On re-exposure, the immune system reacts immediately and triggers a defence response — the allergic reaction (e.g. sneezing, itching, skin redness).

An allergy is therefore a misregulation of the immune system: it responds to a harmless allergen as though it were a dangerous invader.

Important: The diagnosis and treatment of allergies is a matter for medical professionals. If you suspect an allergy, please consult your doctor or an allergist.

What can a weakened immune system indicate?

A weakened immune system can manifest in various ways. Possible signs that warrant a visit to the doctor:

• Frequent or prolonged colds and infections
• Poorly healing wounds
• Persistent fatigue without an identifiable cause
• Recurring inflammation

These signs can have many causes. A doctor can determine through targeted examination whether a change in immune function is present and what the cause might be.

Factors that can influence immune function:

• Nutrient deficiency (e.g. vitamin D, iron, zinc)
• Persistent sleep deprivation
• Chronic stress
• Lack of physical activity
• Excessive alcohol and nicotine consumption
• Certain illnesses or medications

References

[1] Janeway CA et al. (2001). Immunobiology: The Immune System in Health and Disease. 5th edition. Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK10757/
[2] Dirnagl U et al. (2007). Stroke-induced immunodepression: experimental evidence and clinical relevance. Stroke, 38(2 Suppl):770–773. https://pubmed.ncbi.nlm.nih.gov/17261736/
[3] University of Basel, TROVE2 gene study (2020). Original publication available on request.
[4] Elenkov IJ, Chrousos GP (2002). Stress hormones, proinflammatory and antiinflammatory cytokines, and autoimmunity. Annals of the New York Academy of Sciences, 966:290–303. https://pubmed.ncbi.nlm.nih.gov/12114286/
[5] Arcavi L, Benowitz NL (2004). Cigarette smoking and infection. Archives of Internal Medicine, 164(20):2206–2216. https://pubmed.ncbi.nlm.nih.gov/15534156/
[6] Mehta H et al. (2008). Smoking and immunity. Current Opinion in Pulmonary Medicine, 14(4):291–295.
[7] Szabo G, Saha B (2015). Alcohol's effect on host defense. Alcohol Research, 37(2):159–170.https://pubmed.ncbi.nlm.nih.gov/26695755/
[8] Vighi G et al. (2008). Allergy and the gastrointestinal system. Clinical and Experimental Immunology, 153(Suppl 1):3–6. https://pubmed.ncbi.nlm.nih.gov/18721321/
[9] Besedovsky L et al. (2019). The sleep-immune crosstalk in health and disease. Physiological Reviews, 99(3):1325–1380. https://pubmed.ncbi.nlm.nih.gov/30920354/ 

Editorial note: This guide reflects the current general state of scientific knowledge. It does not replace individual medical advice. For health concerns or questions about food supplements, please consult a doctor or pharmacist.