Immune system

The immune system protects the body from invaders and pollutants. To do this, it uses various defense mechanisms. How do the defenses work? What is part of the immune system? How can you strengthen the immune system? You can find out all about it here!

What is the immune system?

The immune system is the human defense system against foreign substances and germs. This is important because the body is in constant exchange with its environment. And many bacteria, viruses, fungi, and parasites exist. If they penetrate the body, they can sometimes cause life-threatening infections.

But also pollutants – for example, air – can damage the body. The immune system’s job is to prevent infection, fight off unwanted invaders, and eliminate toxic substances. The defense system comprises several organs, various cells, and proteins.

What is the structure of the immune system?

The structure of the immune system is complex. It consists of numerous components. The immune system can only protect the body from pathogens if all components work together correctly.

Organs of the immune system

In addition to the blood, the organs of the immune system include the so-called lymphatic system with the lymphatic organs. In addition, the skin and mucous membranes offer essential protection against substances and intruders from the outside.

Organs of the immune system

For example, skin and mucous membranes are the first essential barriers against bacteria, viruses, and fungi on the entire body. They are like a mechanical protective wall that shields the body from the outside world.

Other defense mechanisms also support the work of the immune system:

  • Bacteria-inhibiting substances (e.g., enzymes in saliva, urine, or tears) stop foreign invaders.
  • Mucus in the respiratory tract ensures that inhaled pollutants initially stick and are transported out again by the movement of the cilia.
  • Stomach acid destroys most pathogens that enter the body through food.
  • Beneficial microorganisms colonize the skin and mucous membranes (e.g., the microbiome of the intestinal flora) and fight off pathogens.
  • Reflexes (coughing, sneezing) also protect against pathogens.

Primary lymphatic organs

The lymphatic system consists of the lymphatic vessels and the primary and secondary lymphatic organs. The primary lymphoid organs form specific defense cells, the lymphocytes. These organs include:

  • Bone marrow: central organ of the immune system inside the bones, in which the blood cells are formed and, for the most part, also mature – except for the immature T-lymphocytes
  • Thymus: Organ located above the pericardium where T-cells mature

Secondary Lymphatic Organs

In contrast to the primary lymphatic organs, an actual immune defense occurs in the secondary ones. The mature defense cells migrate from their formation to where they develop further depending on the pathogen and pollutant and fend off the invaders. These immune system organs include:

  • Spleen: Foreign substances (antigens) enter the organ in the upper left abdomen via the blood
  • Lymph nodes: Antigens usually get there via the lymph from the lymphatic vessels
  • Mucosa-associated lymphoid tissue (MALT): The surface of the tissue establishes contact between foreign substances and defense cells, which then take up the fight.
    • Tonsils ( NALT  = Nasal-Pharyngeal-Associated Lymphoid Tissue), the, e.g., the palatine or pharyngeal tonsils
    • Lymphatic tissue in the gut (GALT = Gut-Associated Lymphoid Tissue), such as the appendix and the Peyer’s patches in the small intestine
    • Immune tissue in the airways (BALT = Bronchus-Associated Lymphoid Tissue)
    • Lymphatic tissue in the urinary tract

The thymus is a small gland on the chest’s left side. It’s a part of the lymphatic system, though it’s not generally considered part of the immune system. It’s a “thymic organ” because it produces T-cells, a type of white blood cell that fights infection and disease. The thymus gland grows inside us for the first few decades of our lives. It’s then released into the thymus sac, where it matures into a fully developed gland. It’s usually not a problem, but if part or all of the gland is removed or injured, you’re susceptible to certain types of infection—including thymoma.

Find out here why the thymus is no longer active in adults.

The heart is a muscle, so it makes sense that heart disease impacts the right side of the heart while strokes affect the left side. But that’s only the beginning. The spleen is around 156 to 175 g, a pink, the butterfly-shaped organ that sits between your stomach and liver in your upper abdomen. It’s part of the lymphatic system responsible for getting rid of waste.

The spleen (splen, lien) is the largest lymphatic organ in the body. Read more about their location, anatomy, and essential spleen diseases!

Immune cells

Numerous cells are involved in the immune system. They are called white blood cells, medically leukocytes. The immune system’s various cells communicate directly via unique surface markers or soluble messenger substances such as the so-called cytokines.


Granulocytes belong to the first wave of defense. About 40 to 60 percent of leukocytes are granulocytes. Among other things, they swim in the blood and can also leave the bloodstream and migrate into the tissue. The granulocytes are part of the non-specific immune system.

Physicians distinguish:


The lymphocytes play an essential role in the acquired, specific defense. They also form the so-called immune memory, the basis for the continued protection of vaccinations. Experts divide the lymphocytes into:

B cells (B lymphocytes)

B cells originate in the bone marrow. This is where the term B-cells comes from – from “bone marrow.” From there, they migrate to lymphatic tissue, where they are activated. There and in the blood, they then encounter foreign substances. As mature plasma cells, they produce antibodies. These, in turn, initiate the destruction of the intruder in various ways.

T cells (T lymphocytes)

T cells are white blood cells that are involved in adaptive immune defense. The T cells originate in the bone marrow and then migrate to the thymus (hence the T). There, the defense cells mature before they circulate between the lymphatic tissue and the blood like the B cells.

There are two main types:

  • T helper cells, also known as CD4+ T cells, activate the B lymphocytes via messenger substances and thus initiate the specific defense. They also include the regulatory T cells, which help prevent or end excessive immune reactions.
  • T killer cells are also CD8+ T cells or cytotoxic T lymphocytes. They recognize virus-infected cells or tumor cells and destroy them.

Memory B cells/memory

T cells Some B and T lymphocytes develop into memory cells after the first contact with the pathogen. If the same pathogen invades the body later, the specific immune system “remembers” it. This immune memory allows it to react faster and initiate the appropriate immune response.

This principle is also used for vaccinations. The generally harmless vaccine mimics the first contact with the pathogen. As a result, specific antibodies and immunological memory develop. If the immune system encounters the “real” germ in the future, it can be warded off quickly and effectively.

Natural Killer (NK) Cells

Some scientists regard Natural Killer (NK) Cells as a subtype of lymphocytes, others as a separate line of cells. Unlike B and T lymphocytes, they cannot recognize specific antigens. In addition, the NK cells are immediately ready to defend themselves. Therefore, they belong to the non-specific immune system. They recognize and kill virus-infected and malignant cells.


Monocytes are very large leukocytes. They also come from the bone marrow and can swim freely in the blood. Or they develop into so-called macrophages when they leave the bloodstream and migrate into the tissue.

As part of their defense work, the monocytes or macrophages “devour” bacteria and other microorganisms, cell debris, and other particles (phagocytosis) to dissolve or store them. This group is therefore also called scavenger cells.

They not only “eat up” but also attract other immune cells via messenger substances. They also present parts of the engulfed pathogens to the specific lymphocytes (antigen presentation). In addition, they play a crucial role in inflammatory reactions (cytokine release) and activate the complement system.

Dendritic cells (DCs)

For lymphocytes to develop further and become active, they need contact with antigens. Only some B cells can recognize them directly. T-lymphocytes, on the other hand, need other cells for this. These are the so-called antigen-presenting cells.

In addition to macrophages and B lymphocytes, so-called dendritic cells are also included. They have their origin in the bone marrow and are located in a wide variety of tissue types, for example, in the skin. There they “wait” with their long cell extensions for foreign substances that they can absorb, process, and present as foreign antigens on their surface.

The immune system is the body’s first line of defense against infection, so it must be able to function correctly. When the immune system is working properly, white blood cells circulate throughout the body, patrolling for foreign invaders. In conjunction with macrophages and dendritic cells, white blood cells are involved in the immune response to pathogens. However, if the immune system is compromised, white blood cells cannot do their job as effectively as they should. Leukocytes are white blood cells that are an essential part of the immune system.

Leukocytes are blood cells that are responsible for fighting off infections. Without them, the immune system does not work. Please read all about it!

Humorous defense

Doctors call the fight against pathogens using defense cells cellular immune defense. There are also so-called humoral mechanisms. They are based on special proteins. These can fight invaders directly. They also initiate and strengthen other immune reactions. The humoral immune response is part of the innate defense system.

Complement system

The so-called complement system is a defense mechanism of the innate immune system. It consists of different proteins, the complement factors. These come from the liver and float around in the blood.

You have three essential tasks:

  • Direct Defense: The complement system can directly destroy pathogens.
  • Marking pathogens: The complement factors can keep invaders (opsonization). As a result, scavenger cells can recognize and devour the germs more easily (phagocytosis).
  • Intensification of inflammation: the proteins attract more immune cells and make the blood vessels more permeable – one reason inflamed tissue swells.


Acute-phase proteins

Macrophages and dendritic cells release certain messenger substances (cytokines) during the first pathogen defense. This not only attracts other immune cells. They also trigger the so-called acute phase reaction in the liver. The liver cells then produce specific proteins in increased amounts.

These acute-phase proteins mark pathogens, among other things, so that scavenger cells can better recognize and absorb them. Some of the proteins can also activate the complement system.

In addition to the immune cells, doctors can also determine acute phase proteins in the blood. Well-known representatives are ferritin and the C-reactive protein (CRP).

Immune system cytokines

These proteins are particular messenger substances. They are produced by immune cells. Known cytokines (cytokines) are interleukins, interferons, or tumor necrosis factors (e.g., TNF-alpha). They have a wide variety of functions. As chemokines, they attract other immune cells, for example. They also regulate the proliferation of immune cells and control their further development.

How does the immune system work?

The work of the immune system begins as soon as a pathogen has penetrated the organism, for example with a small skin injury.

Non-specific immune defense

The non-specific immune system is already present at birth. It is therefore also called the natural or innate immune system. As the first instance in defensive combat, it can react quickly to foreign substances.

However, it can hardly distinguish between the different invaders. Therefore, the non-specific immune defense is often not sufficiently effective and only partially able to prevent the spread of some pathogens in the body.

Various components belong to the system of non-specific defense:

  • skin and mucous membranes
  • Body fluids (e.g., saliva, mucus, urine, stomach acid)
  • Local protective mechanisms (e.g., cilia)
  • Natural flora (e.g., bacteria in the intestines or on the skin)
  • Defense cells (e.g., monocytes, granulocytes, NK cells)
  • Proteins (e.g., acute phase proteins, cytokines, complement factors)

Specific immune defense

Since the non-specific defense system is often not sufficient, the specific immune defense is so important, also known as the adaptive or acquired immune system. It is primarily triggered by the antigen-presenting cells. The particular defense cells can then take targeted action against specific pathogens.

However, the acquired immune system needs time, often hours and days, to develop enough clout. It also creates the so-called immune memory for this: If the infection with the same pathogen occurs again, the immune system can react more quickly.

Various immune cells and tissues work together in the specific immune system to fight off pathogens and foreign substances. This includes:

  • T cells
  • B cells (as plasma cells producing antibodies)

Course of the immune response

For the immune system to be able to react to an intruder, it must first be recognized.

Phase 1: Initial response to intrusion

Once a pollutant or germ has overcome the first barrier, it enters the body. This can happen, for example, through a skin injury. This stimulus first activates cells of the non-specific immune system, such as macrophages and granulocytes.

Phase 2: "Examine" foreign substances and fight them

Every foreign substance or pathogen has characteristics such as proteins, carbohydrates, and fats on its surface that the body recognizes as foreign. The non-specific defense cells react to particular “patterns” on the surface of the foreign substance, so-called Pathogen Associated Molecular Patterns, PAMPs for short.

Then they pour out different substances. These can, for example, directly destroy the pathogen. Other substances increase the function of the immune cells or call up new ones.

Phase 3: Identify pathogens specifically

The so-called antigens are much more specific than the PAMPs. The foreign antigens are primarily proteins but can also consist of fat or sugar molecules. A PAMP consists of several antigens. These mobilize the specific defense cells that can take targeted action against individual pathogens.

B cells can either bind directly to appropriate antigens or attach to antigen-presenting cells (APC). T lymphocytes always need the help of APC. In both cases, the principle works like a lock that only fits a specific key. 

Phase 4(a): T cells become active

They become active when the T lymphocytes have docked to the appropriate antigen. Messenger substances, the cytokines, stimulate the T cells, among other things, to divide. In this way, only the T cells that match the pathogen multiply. The immune response is thus “tailor-made.”

Phase 4(b): B cells position themselves

The situation is similar with the B lymphocytes. Once they have bound to antigens, they, in turn, present them on their surface themselves. This is where T helper cells come into play: once they have docked there, they use messenger substances to give the B cells the signal to multiply.
Two types of B cells are formed. The B memory cells for protection against renewed, future diseases by the same pathogen and the plasma cells.

Phase 5: Antibody production

The plasma cells produce matching antibodies, also called immunoglobulins. These are particular proteins for the immune system. Every intruder gets their “own” antibodies.

Phase 6: The antibodies work

The antibodies attach firmly to the pathogen’s antigen, for example, to components of the envelope of bacteria or viruses.

This has several advantages:

  • Opsonization: Antibodies “mark” the pathogen for other defense cells. For example, scavenger cells recognize the invaders more easily if they are peppered with antibodies.
  • Neutralization: Antibodies can, for example, neutralize the toxins of invading germs. If the immunoglobulins bind to viruses, they prevent the viruses from penetrating human cells. As a result, they can no longer multiply.
  • Complement activation: The antigen-antibody complexes also activate the complement system. This leads to the destruction of pathogens or affected cells. The complement system also attracts other immune cells and marks pathogens. It thus connects the non-specific with the specific immune system.


Weak immune system

In some cases, the immune system is no longer as powerful, and the body is more susceptible to infections. Many factors can cause a weak defense. In many cases, lifestyle plays a role. This can usually be changed positively. In some cases, however, there is a disease behind it.

Causes of a weak immune system

Reasons for a weakened immune system are, for example:

Signs of a weak immune system

If the defense is weakened, invaders have an easier time. Pathogens can penetrate the body more efficiently, multiply and spread there. The result: you get sick more often.

In addition to this susceptibility to infections, a weakened immune system often leads to general symptoms. These include, for example:

How do you recognize a pathological susceptibility to infection? What are the possible causes? What to do about it? Read the answers here!

What are autoimmune diseases?

In an autoimmune disease, the immune system attacks healthy, endogenous tissue. Autoimmune diseases can attack any tissue and organs.

The body’s natural defense mechanism is the immune system designed to protect us from harmful substances. Innate immunity is the body’s ability to resist infection and disease, while adaptive immunity is the body’s reaction to an invading pathogen and develops over time as we develop antibodies. An autoimmune disease is where the body’s defense system creates a negative response towards itself, causing autoimmune disease symptoms like indigestion, hair loss, joint pain, and more.

You can find out which autoimmune diseases there are, what symptoms they have and how they are treated in the topic special.

How can you strengthen the immune system?

Above all, a healthy lifestyle contributes enormously to the reliable functioning of the immune system. You can naturally strengthen your immune system simply by exercising regularly. A balanced diet is also an important cornerstone. It usually provides the body with all the essential components necessary for the smooth functioning of the immune system.

These include vitamins such as vitamins A, B6, B9 ( folic acid ), C, and E, as well as minerals and trace elements such as selenium or zinc. But vaccinations also basically strengthen the natural defenses. They set off defense reactions that also take place when fighting actual pathogens: the immune system forms specific antibodies and memory cells. The vaccination “trains” the immune system to a certain extent for emergencies. 

Herbal remedies can also support the immune system. Homeopathy is also said to be able to strengthen the immune system. However, their specific effectiveness is scientifically disputed and has not been proven by studies. Further tips for a healthy immune system are: Relax, restful sleep and drink enough water.

Healthy nutrition, plenty of exercises, stress reduction, garlic & Co.: Find out how you can strengthen or build up your immune system here!

Measles, flu, or hepatitis: which vaccinations are important? When do I need to refresh them? What vaccinations do children and pregnant women need? You can find out what vaccinations there are, how they work precisely, and which vaccinations should be given when the topic is unique.

the immune system in children

There are apparent differences between the immune system in children and adults. Nevertheless, newborns already have all the critical primary defenses. From then on, the immune system continues to develop – for example, by constantly encountering new pathogens.

Children, especially at daycare or kindergarten, constantly feel ill. Numerous germs move back and forth through the close contact between the playing children. Nevertheless, parents should not wrap their children in cotton wool: This is the only way for the immune system to “learn” and develop an immune memory.

Regardless, you can strengthen your child’s natural defenses. For example, make sure you have a healthy diet right from the start. And motivate your children to get out in the fresh air and exercise.

How do they strengthen children’s immune systems? How does it differ from the adult body defenses? Read more about this here!

More about the immune system

There are apparent differences between the immune system in children and adults. Nevertheless, newborns already have all the critical primary defenses. From then on, the immune system continues to develop – for example, by constantly encountering new pathogens.

Children, especially at daycare or kindergarten, constantly feel ill. Numerous germs move back and forth through the close contact between the playing children. Nevertheless, parents should not wrap their children in cotton wool: This is the only way for the immune system to “learn” and develop an immune memory.

Regardless, you can strengthen your child’s natural defenses. For example, make sure you have a healthy diet right from the start. And motivate your children to get out in the fresh air and exercise.

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Scientific standards:

This text corresponds to the requirements of specialist medical literature, medical guidelines, and current studies and has been checked by medical professionals.