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Everything you need to know about a healthy immune system
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Dr. Truls Marius Aarra
Nov 26, 2025
Reading time: 14 minutes14 minutes
The immune system works like your body’s own organizing team. It keeps everything in balance and makes sure all processes run smoothly. Many organs and cells work together in the background without you even noticing.
This article gives you a simple explanation of how the immune system is built, how its two defense lines work, and what can strengthen or weaken it in daily life. You’ll get a clear picture of why a well functioning immune system matters for your overall wellbeing and how you can support it.
Our bodies are constantly under attack: viruses, bacteria, and fungi lurk everywhere. Without our immune system, the body's own defense system, we would be defenseless against them. It acts as the body's police force, so to speak, preventing infectious agents from entering the body from outside and making it sick.
The human body's immune system includes cells and organs, each of which performs a specific task in this complex defense mechanism. These include, among others:
Skin and mucous membranes in the mouth, throat, nose, and intestines: Pathogens enter the body through these areas, where the first defense reactions take place in healthy individuals.
Tonsils: These act as the first line of defense against external pathogens.
Bone marrow: This is where the precursors of defense cells are produced, as well as a small number of defense cells themselves.
Thymus: This glandular tissue is located directly above the heart and is a so-called primary lymphatic organ, i.e., part of the lymphatic system. T cells (defense cells) mature fully in the thymus.
Spleen: This organ performs three functions within the immune system. It supports the proliferation of lymphocytes (white blood cells), stores defense cells that are then available to the body when immune defense is needed, and removes aged red blood cells.
Lymph nodes and lymphatic vessels: These are both a transport route for immune cells and antibodies and a collection point for both.
The two lines of defense of the immune system against pathogens and bacteria
The body's own defense system (immune system) is based on two pillars or two lines of defense:
Adaptive or non-specific immune system: This is innate in humans, i.e. it is born with it. In medicine, it is often referred to as thenatural immune system.
Specific immune system: The human body must first develop this second line of defense. To do this, it must first come into contact with the pathogens against which the body can then form antibodies. For this reason, it is also called thelearned or acquired immune system.
For a long time, medical professionals did not understand exactly how these two lines of defense work. The first insights into the specific immune system and how it works came with the discovery of so-called dendritic cells in 1973 by Ralph Steinmann. Dendritic cells act as an early warning system, alerting the acquired immune system as soon as foreign substances enter the body. Insights into the early warning system of the adaptive (innate) immune system, on the other hand, were provided by the research work of Luxembourg-born biologist Jules Hoffmann and American Bruce Beutler (immunologist and geneticist) in 1990.
Mechanical and chemical barriers are protective agents of the adaptive immune system
The adaptive (innate) immune system is the first to respond when pathogens or germs find their way into the body. It identifies many foreign bodies and pathogens upon first contact. It is divided into
anexternal protective function, known as themechanical barrier, and
aninternal protective mechanism, known as thechemical barrier.
Themechanical barrier includes:
Skin and
Mucous membranes.
They protect the human body purely mechanically.
The mechanical barrier is supported by the so-called chemical barrier. These are chemical substances and body fluids that block the deposition of viruses and bacteria or flush them directly out of the body (transport them away).
The chemical barrier includes, for example
eye fluid,
saliva,
urine,
vaginal fluid,
gastric juice, but also
the cilia of the bronchi or the intestinal muscles, which, through their respective movements, ensure that no germs and bacteria can settle in the bronchi or intestines.
The tools used by the chemical barrier are the so-called natural phagocytes and killer cells. Both are active in cellular defense and directly absorb pathogens and foreign bodies, destroying them or transporting them away.
Phagocytes belong to the white blood cells (leukocytes). They digest the pathogen – hence their name. What remains of the pathogen is released to the cell surface, where the specific immune system takes over the further immune defense.
Killer cells, on the other hand, are primarily active against viruses and tumor-like cells. When they recognize such cell types, they produce special cell toxins that destroy viruses or tumor cells.
The specific immune system fights specific pathogens in a targeted manner
In order to do its job well, the specific immune system must first get to know the pathogen. It develops indirect confrontation with the pathogen and responds by producing special defense cells tailored to this pathogen. This means that the immune response of the acquired immune system takes longer when it first comes into contact with the pathogen. However, upon second contact, this system works much more effectively because it already knows the pathogen.
This specific immune system is also the reason why the human body is immune to certain diseases and why the symptoms are significantly weaker or even completely absent upon second contact with the pathogen.
The acquired immune system works with two types of cells:
T lymphocytes (T cells) and
B lymphocytes (B cells).
T cells: The human body produces T cells in the bone marrow. From there, they migrate to the thymus to mature (which is why they are called T cells) and then settle in the tissue. In their fight against external pathogens, T cellsperform three different tasksas
T helper cells – trigger the body's immune response
T killer cells – recognize and destroy infected cells (including tumor cells)
memory cells– ensure that the specific immune system can immediately identify the specific pathogen upon next contact and react quickly
B cells: These are also produced in the bone marrow, but remain there to mature. Similar to T cells, B cells are named after the place where they mature. The English term for bone marrow is ‘bone marrow’, hence the name B cells.
B cells also always match a specific pathogen and are only formed when the body comes into contact with that pathogen. B cells are also activated by T helper cells. As soon as the pathogen invading the body matches the B cell, the T cells sound the alarm, the B cells multiply rapidly and transform into plasma cells. These, in turn, produce numerous antibodies (immunoglobulins/proteins) against the specific pathogen within a very short time, rendering it harmless.
How do the two lines of defense differ from each other?
While the innate immune system figuratively shoots at anything foreign that crosses the body's barrier, the acquired immune system specializes in specific pathogens. With specially tailored antibodies – known as T and/or B cells – it can target this specific pathogen and is therefore more efficient, even if it takes longer to mount an initial immune response and thus works more slowly than the innate immune system when first encountering the pathogen.
The immune system can be strengthened or weakened by a variety of factors
Many external factors can influence the immune system both positively and negatively. For example, an unbalanced diet, nicotine, and alcohol, as well as everyday stress, can have a negative effect on your immune system. A balanced diet and the targeted intake of micronutrients such as zinc or vitamin D can strengthen the body's own defenses.
As part of the immune system, the intestine has a major influence on the body's own defenses.
The intestine, which is around six to nine meters long and has a total surface area of 400 square meters, is responsible for approximately 80 percent of the body's total immune defense. The intestine must decide what is allowed to pass through the intestinal barrier into the bloodstream. Here, good bacteria must be distinguished from bad bacteria, and pathogens must be denied access to the bloodstream. These facts make it clear what a major influence the intestine has on the body's own defenses as part of the immune system.
What is the significance of the brain for the immune system?
Until now, scientists have assumed that the immune system of the brain is separate from that of the rest of the body. This separation is achieved by the blood-brain barrier. It separates the immune cells into those that are active exclusively in the brain and those that are active exclusively in the rest of the body. The latest scientific studies have now shown that the two areas of the body are not quite as strictly separated from each other as previously thought and, above all, that the brain does indeed influence the immune system.
For example, people with brain damage caused by a stroke are more susceptible to infections, especially if the brain damage mainly affects the left hemisphere. The study participants showed a strikingly high incidence of infections caused by intestinal bacteria. This leads to the conclusion that damage to the left side of the brain has a particularly negative effect on the immune system of the intestine.
Another study conducted at the University of Basel has shown that certain human genes influence both the immune system and various areas and functions of the brain. The focus here was on a variant of the TROVE2 gene, which plays a role in both immunological diseases and memory function.
Prolonged stress can cause lasting damage to the immune system
The autonomic nervous system and the immune system are closely connected. The two are linked by the sympathetic and parasympathetic nervous systems. These two nerve strands run from the brain through the entire human body. They also supply some organs and cells, including the spleen and lymph nodes, which contain parts of the immune system.
Certain nerve stimuli—including stress—can thus travel directly from the brain to the immune cells. In this way, the brain can influence the immune system both positively and negatively. In this context, stress is one of the negative influencing factors.
Alcohol and cigarettes weaken the immune system
Smokers are more susceptible to infections. This is because nicotine inhibits the formation of immunoglobulins (antibodies that defend against foreign substances and pathogens). For this reason, wound healing in smokers is significantly slower than in non-smokers. Smokers' wounds also become inflamed more quickly.
These symptoms become more pronounced the longer the person has been smoking and if arteriosclerotic vascular changes have already developed, which also affects the blood circulation in the skin, further negatively impacting the wound healing process.
Alcohol is similarly harmful to the immune system. Doctors know that people who frequently consume excessive amounts of alcohol are more prone to pneumonia. Studies have now been able to explain this observation.
Among other things, a study conducted at Loyola University Chicago has shown that excessive alcohol consumption initially leads to an overreaction of the immune system. The production of immune cells increases massively. A few hours later, however, the opposite effect sets in. The blood of the study participants contained significantly fewer killer cells, but a high number of cells that sustainably suppress the immune system.
These micronutrients play key roles in immune competence
Scientific studies show that supplying our bodies with micronutrients (vitamins, minerals, trace elements, and secondary plant substances) can have a positive effect on the immune system. The following micronutrients, among others, are particularly important for the immune system:
The importance of a healthy and balanced diet for the immune system
One of the most important prerequisites for a healthy immune system is therefore a balanced diet. To ensure that your immune system can function optimally, you should therefore make sure that you
provide your body withsufficient amounts of vitamins, minerals, trace elements, and secondary plant substances every day through your diet or with the help of dietary supplements.
Avoid being both overweight and underweight, as being overweight increases susceptibility to infection. Being underweight, on the other hand, is usually associated with a nutrient deficiency, which also has a negative effect on the immune system.
Avoid a high-fat diet, as this could suppress the function of your immune system and thus increase your susceptibility to infection.
Omega-3 is found in high quantities in flaxseed and rapeseed oil, among other things, but also in fish.
How to train your learned immune system and strengthen your body's defenses
The immune system is a complex system involving various organs, cells, tissues, and signaling substances. It is constantly on alert to ward off any external threats as quickly as possible. In order for this defense mechanism to function smoothly, the immune system relies onhelp and reinforcement from outside. You can support your immune system in its work with, among other things:
a balanced diet
medications and dietary supplements
tried-and-tested home remedies
Can medications support and strengthen the immune system?
Especially in times of high stress, such as during
stress
illness or even
competitive sports
you are often unable to ensure that your body is supplied with micronutrients. In these situations, it can be helpful to support the immune system with supplements designed to strengthen it. These supplements should primarily contain the following vitamins and minerals, which are important for the immune system:
To build up the specific immune system, children's immune defenses should also be strengthened.
In children, the specific immune system is still in the development phase. Young children in particular come into contact with many foreign substances and pathogens for the first time in their lives. This means that a lot of immune work is required by their small organisms. With a few simple aids, you can support your child in developing a well-functioning specific immune system.
Make sure your child gets plenty of exercise, preferably outdoors in the fresh air.
Keep your child away from stress as much as possible.
Ensure they get enough sleep.
Make sure they have a balanced diet and an adequate supply of micronutrients.
From the age of two, your child can take vitamin supplements suitable for children to complement a balanced diet.
Suitable home remedies for strengthening the immune system
In addition to a healthy, balanced, and nutrient-rich diet, and possibly also taking vitamin supplements and dietary supplements, you can also use tried-and-tested home remedies to strengthen your immune system.
Our grandmothers already knew that
spicy food can boost the immune system and fight off viruses.
Hot, nutrient-richchicken broth can strengthen your immune system and prevent infections, especially in winter.
The antibacterial properties of certainessential plant oils, such as ginger, thyme, or pepper, can protect the body from attacks by viruses and bacteria.
Regular nasal rinsing with salt water, preferably using a sea salt solution, can help flush viruses and bacteria out of your nose so that they cannot enter your body in the first place.
How does an allergy affect the immune system?
Your immune system is designed to protect you from harmful external attacks – from pollutants or pathogens. However, this defense mechanism can sometimes overreact and fight substances that are not actually harmful to the human body. This overreaction of the immune system is called an allergy.
How an allergy can develop,
what exactly happens in the human body during an allergy,
whether a weak immune system can trigger an allergy,
what the most common triggers of allergies are, and
when allergy symptoms usually occur
Possible consequences of a weak immune system for your health
The most common consequence of a weak immune system is increased susceptibility to infections caused by viruses or bacteria. People with a weak immune system are therefore more likely to suffer from colds, flu-like infections, or influenza.
But that's not all: a weak immune system can have further effects on your general state of health and lead to the following symptoms and illnesses, among others:
psychological symptoms such as difficulty concentrating
poor or slow wound healing
gum inflammation
urinary tract infection
gastrointestinal infection
pneumonia
scarlet fever
Frequently asked questions about the immune system
There is no single miracle cure. Your immune system responds most strongly to your everyday life. Sleep, exercise, nutrition, and stress levels have a greater effect than any pill. If you want to name something that really counts, it's good sleep.
