Functions of the immune system: How and why

• The function of the immune system is to protect our body from all kinds of diseases.
• It consists of two defence structures, the innate and the acquired immune system.
• The immune system is based on cellular and humoral defence mechanisms. Cells are, for example, the macrophages (phagocytes). Humoral defence refers to body fluids (e.g. blood, lymph) and their constituents (e.g. cytokines).
• The cellular and humoral immune systems are divided into a specific and a non-specific part. At the same time, both systems work together, build on each other and complement each other.
• We can support the work of our immune system by nurturing and strengthening it.

Without the body's own defences, we would not be able to fight harmful organisms or substances from outside or threatening changes within our body and thus prevent an infection or disease or at least limit the damage.

The immune system can detect danger signals emanating from our own cells, e.g. cell damage caused by cancer, sunburn or injury. It also registers and responds to signals that viruses and bacteria emit when they enter the body.

The main functions of the immune system are:

• Combating and rendering harmless disease-causing germs, i.e. bacteria, fungi, parasites and viruses.
• Recognising and neutralising harmful substances from our environment.
• Catching or reversing pathogenic physical changes, e.g. the uncontrolled proliferation of cells in cancer.

The immune system is activated by foreign proteins, so-called antigens. The word antigen thus refers to a protein that triggers a defence reaction of the immune system: antibodies are formed. Such proteins are found, for example, on the surface of bacteria, fungi and viruses.

These antigens dock onto special receptors of cells of the immune system, which triggers a whole cascade of processes:

• The immune system has many different types of cells that circulate in the body or are incorporated into a tissue. The most important of these are the leukocytes, which are able to move around the body independently and provide a large part of the body's immune defence. Leukocytes are divided into three groups: Granulocytes, lymphocytes and monocytes.
• The lymphocytes are decisive for the immune response. They are the basis for the acquired immune system and the immunological memory. About a trillion of these cells circulate in the body of every human being in constant search of possible pathogens. There are two types of lymphocytes: B cells produce antibodies that attack pathogens and harmful substances. T cells destroy infected or degenerated cells.
• In the spleen, the T lymphocytes that have migrated from the spinal cord are imprinted and selected (see lymphatic system). They can then differentiate between the body's own and foreign tissue and identify and attack bacteria, viruses, parasites or tumour cells. The body's own cells are spared.
• Each cell type of the immune system has a specific task and can communicate with other cell types.
• Infectious microbes are recognised as a danger by their molecular patterns.
• Other molecular patterns enable the body's own defence system to differentiate between healthy and diseased cells, for example to recognise infected cells or cells damaged by other influences.
• Subgroups of T cells are killer T cells, which kill pathogens, and helper T cells, which help decide what immune response the body should give to a particular pathogen.

What does the innate/non-specific immune system do?

We come into the world with part of our immune system already in place. It is fixed in the genetic material and cannot adapt, i.e. it is not capable of learning like the acquired immune system. But it's not to be underestimated: It reacts extremely quickly and efficiently to organisms recognised as foreign. Cells circulating in the body recognise the type of pathogen within minutes and alert specialised immune cells. This detects the characteristic structures of the various micro-organisms and destroys them. Most threats are thus identified in the shortest possible time and defeated within a few hours.

The innate/non-specific immune system has the following components:

• Barriers that prevent the penetration of harmful substances and organisms.
• Defence cells in our organs.
• Defence cells and proteins that float in the blood plasma and also have defence functions or serve as messenger substances.

How does the non-specific defence work?

• Leukocytes (see above) are a very important component of the non-specific as well as the specific defence. In the innate immune system, macrophages in particular play a major role, as they "eat" and dissolve pathogens. Other types of leukocytes are mast cells, which store messenger substances such as histamine and heparin. Eosinophil granulocytes are mainly specialised in fighting parasites and worms. Basophilic granulocytes contain serotonin in addition to histamine and heparin.
• A very important function in innate immunity is performed by NK cells (natural killer cells), which also mature in the bone marrow and then circulate in the blood or accumulate in the spleen. They are a subgroup of the lymphocytes and destroy mainly tumour cells and virus-infected cells.
• The non-specific immune system has other types of cells that are able, for example, to capture bacteria and render them harmless and also destroy tumour cells.
• The skin is the first mechanical barrier of innate defence. Their sebaceous and sweat glands as well as their microbiome (skin flora consisting of non-harmful, mostly bacterial micro-organisms) prevent the penetration and slow down the growth of foreign and potentially harmful micro-organisms.
• Chemical components of the innate defence include interferons (signalling substances involved in the defence against viruses) and interleukins (messenger substances of the immune system). Enzymes such as lysozyme kill micro-organisms.
• The secretion produced by the mucous membranes binds invaders and enables them to be fought.
• Cilia in the respiratory tract and tears take over the removal. The cough reflex is also part of the innate defence.
• The stomach and intestines form a strong defence system: The hydrochloric acid produced in the stomach and enzymes that break down proteins decompose most of the micro-organisms that are foreign to the body. The intestinal microbiome (intestinal flora) fends off infections; almost 80 percent of all cells that produce antibodies live in the intestinal mucosa.
• Just like the intestine, the urinary tract fulfils the task of constantly removing potentially harmful substances and pathogens.

The innate immune defence reliably fends off a large part of infections and diseases.

What does the acquired/specific immune system do? The acquired immune system is adaptive, very complex and responsible for specific threats. The danger from a pathogen must be analysed and recognised by the body. Antibodies are then produced that are specifically tailored to the current disease germ. Once this is neutralised, the specific immune system "remembers" this information. Therefore, when the same pathogen attacks again, the immune response is faster and more effective.

How does specific defence work?

The specific immune response is based on the interaction of two groups of lymphocytes (white blood cells): T and B cells. The cells involved carry precisely fitting receptors on their membrane that recognise very specific antigens. When these receptors attach themselves to the outer shell of a pathogen, the immune reaction is set in motion.

• B lymphocytes originate in the bone marrow. B plasma cells form loads of antibodies against a bacterium or virus by dividing. The long-lived memory B cells are responsible for the formation and again mass production of plasma cells upon re-infection.
• T lymphocytes, like B cells, originate in the bone marrow and then migrate to the thymus gland where they mature. T cells have multiple functions in the complex interplay with B cells.
• T lymphocytes migrate through the organism and monitor the surfaces of the body's cells for changes. Through contact with such a recognition mark (antigen), they are activated and in turn stimulate the B lymphocytes to divide. • T killer cells destroy infected or degenerated cells. T helper cells support the immune response. Finally, regulatory T cells (T suppressors) terminate the immune response.
• Regulatory T cells reduce the risk of autoimmune diseases or allergic reactions by suppressing the immune response in certain situations.

The lymphatic system is an important player in the immune system: a network of tissues and organs that transport waste, toxins and other unwanted substances out of the body. It consists of the lymphatic channels and the lymphatic organs such as lymph nodes, spleen, various tissues in the gastrointestinal tract, pharynx, tonsils and thymus gland.

• The main function of the lymphatic channels is to transport lymph, a clear fluid containing white blood cells.
• Lymph nodes, small bean-shaped structures, are located in many places in the body, such as the armpit, pelvis, abdomen, and groin. They make more cells that fight infections in the body and also store them. If you suffer from an infection, lymph nodes can swell and hurt.
• The spleen contains white blood cells for defence and disposes of old or damaged blood cells.
• Precursors of the important T lymphocytes migrate from the bone marrow via the bloodstream into the thymus gland. There they are prepared for their various functions and mature into fully functional T cells. The thymus is located under the breastbone and consists of two lobules. In newborns, each lobe is about six centimetres long and two centimetres wide. In childhood, it still grows a little and keeps this size until puberty. Later, the thymus tissue is replaced more and more by functionless fatty tissue. After its regression, the lymph nodes and spleen take over its functions.

The efficiency of the immune system is not always the same, but varies under the influence of a lot of factors:

• Time of day matters: The number of immune cells is highest in the evening. • Stress and poor mental health have a negative effect.
• With age, the number of certain immune cells decreases; they also no longer recognise pathogens or pre-damaged body cells as well.
• Lifestyle also always has an impact on the immune system.

The best measures to strengthen the body's defences are relatively simple. Some things, like reducing stress, are more difficult to achieve in today's lifestyles. But you'll help your immune system if you consider the following factors:

• A balanced and varied diet with plenty of vegetables and fruit.
• Drink enough (water).
• Sufficient exercise: half an hour of brisk walking per day already achieves a lot. Active sports are even better.
• Keep your weight at a healthy level.
• Enjoy sunlight in moderation and thus boost the production of vitamin D.
• Avoid overexertion and reduce stress.
• Make sure you get enough sleep.
• Do not smoke, drink alcohol in moderation at most.

You also support your immune system when you reduce sources of infection: If you wash your hands frequently and regularly, pay attention to hygiene when preparing meals and keep your vaccination pass up-to-date, your immune system will be better able to fight against pathogens.

Due to a very rare hereditary disease, children are born from time to time who suffer from the congenital severe immune deficiency SCID ("Severe Combined Immunodeficiency Disorder"). It can be treated by transplanting bone marrow cells from a relative. But without a fully functioning immune system, these babies have no protection against viruses and bacteria and in most cases die very early.

In the 1970s, the case of an American boy who spent his entire life in a sterile plastic isolator and died at the age of twelve after a failed bone marrow transplant caused public attention.

When our body first comes into contact with a pathogenic germ, information about it and how to fight it is stored in the immune system. This allows for quicker detection and countermeasures in the event of a second contact.

Antigens are also found on the cells of our body. But as a rule, the immune system has learned to recognise them as our "own" during the development in the womb and in the first weeks after birth. So they don't trigger an immune response.

However, it can happen that the immune system mistakenly identifies the body's own cells as "foreign" and attacks harmless body cells. This is called an autoimmune reaction (see section "Can the immune system react incorrectly?").

Can the immune system react incorrectly?

Even though the body's defences work tirelessly to protect us from disease, mistakes and malfunctions can still occur. In particular, the development of specific/learned immunity is highly complex and therefore also susceptible to disorders.

Normally, the immune cells deactivate themselves after their work is done. Once a pathogen has been repelled, special combinations of molecules are formed that signal that the danger has passed. The defence only becomes active again when its memory recognises the stored characteristics of the pathogen anew.

However, it can happen that during this process, instead of the "foreign" characteristics, the body's own or body-like characteristics are also incorporated into the immune memory. Then the defence can no longer distinguish between own and foreign cells and is directed against cells of the own body. Such an autoimmune reaction triggers inflammation and organ damage.

• rheumatoid arthritis
• chronic inflammatory bowel diseases such as colitis or Crohn's disease
• diabetes
• Hashimoto's thyroiditis, a chronic inflammation of the thyroid gland
• Lupus erythematosus/systemic lupus; a chronic inflammatory autoimmune disease of the connective tissue
• multiple sclerosis.

These are usually serious diseases that can even be fatal. But in the vast majority of cases, they are also treatable. Symptoms vary widely; common signs include fever and fatigue.

Blood tests can identify the disease. Drugs are used to suppress the immune system at least temporarily, e.g. corticosteroids in rheumatoid arthritis, or to alleviate weaknesses in the immune system, e.g. by infusion of antibodies. However, exactly how autoimmune diseases occur and how they can be treated has not yet been fully researched.

A broader definition of autoimmune reactions also includes allergic asthma, hay fever, and rashes. Here there is no lack of differentiation between "own" and "foreign", but an overreaction against external stimuli occurs. A normally rather harmless substance such as pollen, nuts, moulds, dust or the saliva of pets is perceived as a threat and attacked (too) intensely.