The immune system ( Latin immunis 'untouched, free, pure' ) is the name given to the biological defense system of higher living beings, which prevents tissue damage from pathogens . This body's own defense system removes microorganisms and foreign substances that have penetrated the body and is also able to destroy the body's own cells that have become defective . The immune system is a complex network of different organs , cell types and molecules and the central research subject in immunology .
The immune system is of great importance for the physical integrity of living beings , because practically all organisms are constantly exposed to the influences of the living environment; some of these influences pose a threat: if harmful microorganisms enter the body, this can lead to malfunctions and diseases. Typical pathogens are bacteria , viruses and fungi , as well as unicellular (e.g. protozoa such as plasmodia ) or multicellular parasites (e.g. tapeworms ).
Changes inside the body can also threaten the existence of a living being: If normal body cells lose their healthy function over time, then they usually die and have to be broken down ( necrosis ) or break down themselves ( apoptosis ). In rare cases, they can also degenerate pathologically and lead to the development of cancer .
All living beings therefore have protective functions. Even simple organisms have such defense mechanisms , a so-called innate immune response . This originated very early in the tribal history of living things and has remained largely unchanged since then. The vertebrates also developed a complex, adaptable, so-called adaptive immune defense , which protects them even more effectively against pathogens.
The plant immune response is similar to the innate immune response in animals. Plants do not have an adaptive immune response, i.e. also no T cells or antibodies.
There are two fundamentally different mechanisms of the immune defense, depending on whether this is innate and therefore in a certain way (but see below: bow-tie architecture ) pathogen-unspecific, or whether it is acquired and therefore pathogen-specific.
Innate or unspecific immune defense
The unspecific or innate immune defense (engl. Innate immunity ) developed very early in the tribal history of living things . These include anatomical and physiological barriers such as epithelia , but also cell-mediated resistance through phagocytosis , as well as general inflammatory reactions and the complement system . The innate immune response takes place within minutes, but is determined by genetic information for life.
Adaptive or specific immune defense
The specific or adaptive immune defense , formerly also called the "acquired immune system", developed from the innate immune defense in the course of the phylogenesis of vertebrates. It is characterized by the ability to adapt to new or changed pathogens. As part of this adaptation, the cells of the adaptive immune defense are able to recognize specific structures ( antigens ) of the attacker and to specifically form cellular defense mechanisms and molecular antibodies . In addition to antigen presenting cells (APC) such as dendritic cells, two groups of cells represent the essential elements of adaptive immunity. The T lymphocytes , which on the one hand ensure the cell-mediated immune response and on the other hand support the B lymphocytes , as well as the B lymphocytes themselves that are responsible for humoral immunity, i.e. for those defense measures that are directed against intruders in the body fluids (humores) via secreted antibodies. After infection, specific antibodies and memory cells are retained in order to enable an appropriate defense reaction within a short period of time in the event of renewed contact with the pathogen.
The adaptive immune system does not replace the innate one, but works with it. The various components of the immune system are mutually dependent. The complex immune reaction of the body is only made possible by a well-coordinated interaction of the innate and adaptive immune defense .
The components of the immune system are
- mechanical barriers designed to prevent pests from entering
- Cells such as granulocytes , natural killer cells ( NK cells ) or T lymphocytes . Some of them are grouped together to form specialized organs (→ lymphatic system ).
- Proteins that serve as messenger substances or to ward off pathogens
- mental immune factors .
Mechanical and biochemical barriers
The body's mechanical and biochemical barriers and defense mechanisms are the first line of defense against pathogens. They ensure that the pathogens cannot even penetrate the body or leave it again as quickly as possible:
- Skin - outer layer as a barrier, sebum , sweat and normal flora as a brake on growth for pathogenic microorganisms
- Mucous membrane - binding function of the mucus
- Eyes - function of the removal of tears, antimicrobial enzyme lysozyme fights microorganisms
- Respiratory tract - the binding function of the mucus, the removal function of the cilia
- Oral cavity - antimicrobial enzyme lysozyme in saliva fights microorganisms
- Stomach - Stomach acid (which contains hydrochloric acid ) and protein-degrading enzymes destroy almost all bacteria and microorganisms
- Intestinal defense against infection by the presence of bacteria ( intestinal flora ), evacuation function through constant emptying and the so-called gut - associated immune system ( Gut Associated Lymphoid Tissue, GALT) and antibacterial proteins
- Urinary tract - removal function through constant urine flushing as well as osmotic effects of the high urea concentration
The cells of the immune system circulate in the blood vessels and lymphatic systems and are found in the tissues of the body. If a pathogen penetrates the body, the immune cells can fight it. Neutrophil granulocytes, monocytes / macrophages and dendritic cells can, for example, destroy the pathogen through absorption and digestion ( phagocytosis ) or through the production of immunomodulators and cytokines control the immune reaction of the organism and attract other immune cells to the site of inflammation.
Granulocytes (from the Latin granulum 'granule' ) make up the majority of white blood cells ( leukocytes ). They can leave the bloodstream and migrate into the tissue. Granulocytes have numerous vesicles (called vesicles or granules) in their cytoplasm , which contain aggressive substances with which pathogens can be rendered harmless. Other substances (such as histamine ) play a role in the inflammatory reaction and allergies . The different groups of granulocytes are classified according to their staining reaction in the Giemsa stain .
The neutrophil granulocytes make up 40 to 50 percent of the circulating leukocytes. Activated by cytokines that are secreted from the site of infection, they migrate from the blood vessels into the affected tissue. The granules of the neutrophils contain acid hydrolases, defensins (30% of the content), myeloperoxidase and proteases such as elastase , collagenase , neuraminidase and cathepsin G , among others . This “cocktail” enables the neutrophils to pave their way through the tissue and penetrate to the bacteria. There they are able to destroy pathogens (e.g. bacteria) by phagocytosis, among other things.
Eosinophils make up about 3–5 percent of the cells in the differential blood count . They get their name from the dye eosin , with which they can be colored. Eosinophils are also capable of chemotaxis , i. that is, they can move towards a site of inflammation. Eosinophils contain basic proteins in their granules , for example the major basic protein , which they release after stimulation by antibodies of the IgE class. Eosinophils play an important role in defense against parasites; when infected with parasites, there is therefore a strong increase in eosinophils in the blood. The number of eosinophils in the blood is also increased in the case of allergies, which indicates that the eosinophils also play a - less beneficial - role in this disease.
Basophil granulocytes have numerous coarse, irregular granules that contain histamine and heparin , among others . In the differential blood count, they make up only a small proportion (<2 percent). When their receptors are stimulated by allergens bound to IgE, basophils release toxic mediators such as histamine and platelet activating factor (PAF). However, there is largely a lack of clarity about the physiological significance of the basophils.
Macrophages (giant eating cells) also represent part of the patrol of the immune system. Macrophages mature from monocytes (mononuclear white blood cells = mononuclear leukocytes ) that leave the bloodstream. Macrophages reside in the tissue, where they recognize and eat (phagocytize) any pathogens that have entered. If the pathogens cannot be fought by the macrophages alone, macrophages can activate the adaptive immune system. For this purpose, the absorbed parts of the pathogen are broken down into individual peptides ( epitopes ) inside the macrophages and presented on the surface using MHC-II molecules . The macrophage thus becomes an antigen-presenting cell. The antigens can only then be recognized by T helper cells, which then initiate an adaptive immune response that ultimately leads to the destruction of the pathogen. Macrophages also play a crucial role in combating and eliminating harmful substances and waste products (for example tar from cigarette smoke in the lungs), which is why they are sometimes referred to as "garbage collection from the body".
Natural killer cells
The natural killer cells (NK cells) discovered in 1975 are part of the innate immune system. Although NK cells do not have antigen-specific receptors on their surface, they are counted among the lymphocytes because they share a common precursor cell in the bone marrow.
NK cells are one of the first lines of defense in the fight against infection and cancer because they can destroy infected cells without first being in contact with the pathogen itself. Use them a mechanism that in the 1980s by the Swedish immunologist Klas Kärre was discovered and as a Lack of Self ( English missing self ) is called. Among other things, NK cells recognize the MHC-I complex, which occurs on almost all healthy body cells. If a cell is infected by viruses or if it turns into a tumor cell, the MHC-I complex on the surface may be lost. The finely balanced balance of inhibiting and activating receptor signals is shifted in favor of NK cell activation and the diseased cell succumbs to an immune reaction triggered by NK cells.
Dendritic cells are cells of the immune system that develop from monocytes or T-cell precursors, depending on their type . As scavenger cells (phagocytes) they absorb pathogens, migrate to the next lymph node , and stimulate the adaptive immune defense by presenting the antigens of the pathogen to the T lymphocytes on their surface. One dendritic cell is enough to activate 100 to 3,000 antigen-specific T cells. This makes them more efficient than e.g. B. Monocytes. Dendritic cells also ensure immunological tolerance to self-antigens . They are mainly found in the skin and mucous membranes. Dendritic cells can also interact with B and NK cells.
T-lymphocytes, also called T-cells, arise in the bone marrow from lymphoblasts and migrate to the thymus , where they mature (hence the T, dependent on the thymus). T cells have a T cell receptor (TCR) on their surface , with which each T cell can recognize a specific antigen ( lock and key principle ). In contrast to B lymphocytes, which also recognize free antigens, T cells only recognize antigens that are presented in complex with MHC molecules on the surfaces of the body's own cells. The different types of T cells are divided according to the proteins on their cell membrane , which are also important for the functions of the cells: T helper cells, for example, carry the CD4 protein (the abbreviation CD stands for cluster of differentiation ), the cytotoxic T cells have the CD8 protein on their surface.
T helper cells
The T-helper cells coordinate the immune response. They recognize antigens via their specific T-cell receptor, which are presented to them by the antigen-presenting cells (dendritic cells, macrophages, B-lymphocytes) on MHC-II complexes. This activation causes the T helper cells to divide and release their messenger substances: the lymphokines of the cells of the T H 1 subtype tend to strengthen the cellular immune response, while T H 2 cells stimulate the production of antibodies.
Regulatory T cells
The regulatory T cells , which were first described in the mid-1990s, carry other proteins on their surface in addition to the CD4 receptor ( CD25 , FoxP3 ). Their job is to modulate the immune response. Furthermore, regulatory T cells are presumably responsible for suppressing an excessive immune response to otherwise 'harmless' antigens and developing tolerance towards the body's own structures.
Cytotoxic T cells
The cytotoxic T cells can recognize antigens that are presented to them with the help of the MHC-I complexes - the body's own cells that are infected by pathogens (e.g. viruses) report their condition to the immune system. The cytotoxic T cells then attach themselves to these body cells with their T cell receptors; your CD8 receptor plays a crucial role in this process. If other receptors, for example the CD28 receptor of the cytotoxic T cells, have attached to the foreign protein, the T cells begin to multiply quickly and release substances that cause the infected or pathologically changed cell to die ( so-called apoptosis, programmed cell death).
B lymphocytes, or B cells for short, also belong to the group of leukocytes (white blood cells). The designation “B cells” originally came from their place of formation in the bursa Fabricii in birds. In mammals , the B cells, like all other immune cells, are created in the bone marrow , which is why the letter B was subsequently given the meaning of bone marrow . If a B cell binds to the substance (antigen) that matches its receptor, it can be activated by lymphokines that are released from activated T helper cells. The B cells activated in this way can then develop into antibody-producing plasma cells or into memory cells.
In contrast to T cells, B cells are also able to recognize free antigens and initiate an immune reaction.
The humoral components of the immune system (from humor 'fluid' ) denote various plasma proteins that passively circulate in the blood or in the lymph and tissue fluid. In contrast to the immune cells, they are unable to actively migrate to the site of an infection.
To ward off bacteria, bacterial toxins, viruses or other foreign substances that have penetrated the organism, the B lymphocytes and plasma cells produce tailor-made antibodies that recognize certain proteins or sugar chains (antigens) on the surface of the foreign substances and can attach themselves to them. Antibodies basically have three functions:
- The so-called opsonization . This means that the Fc part (part of the constant chain of the antibody) makes the antigen more “visible” to phagocytes (scavenger cells).
- The so-called complement system is activated by the antigen-antibody complex, which on the one hand acts as opsonin (= substances that opsonize) and on the other hand releases chemotaxins (attractants for cells of the immune system) and forms a so-called MAK (membrane attack complex), which causes holes in cell membranes.
- Antibodies have a direct inactivating effect on the intruder by sticking together and forming large complexes (depending on the antibody class and number of antigen determinants).
The simplest antibodies, those of the so-called IgG class, consist of two identical heavy chains and two identical light chains. The heavy chains are, inter alia, for anchoring the antibody on the surface of granulocytes responsible; the light chains together with the heavy chains form the antigen determinant in the Fab fragment, which is responsible for the recognition of a specific antigen . Through somatic recombination , somatic hypermutation and the combination of different light and heavy chains, antibodies can form more than 100 million different Fab fragments and thus recognize a myriad of different antigens.
The complement system is part of the innate immune response; it consists of a group of over 30 plasma proteins with very different properties. Some of the proteins belonging to the complement system are, for example, proteases, which can bind to microorganisms and damage the intruder's cell walls, thereby destroying the intruder. Other proteins of the complement system, the anaphylatoxins , have a vasodilating effect and promote the inflammatory reaction. Many complement factors can also attract immune cells to the site of infection and are able to activate phagocytes, which the invaders then devour.
The interleukins, which belong to the cytokines, are the body's own messenger substances that are produced by the cells of the immune system. Today we already know a large number of interleukins (IL-1 to IL-35; as of November 2009), each of which acts on very different immune cells - for example, some stimulate leukocytes to grow, mature and divide or ensure that they are activated.
Course of an immune reaction
If pathogens overcome the mechanical barriers with which the body protects itself from infection, the course of the immune reaction depends on whether the immune system has previously had contact with this particular pathogen.
In the case of an initial infection, the immune reaction usually begins with the antigen-presenting cells . B. macrophages or dendritic cells ; As part of the innate immune defense, these cells are able to recognize typical characteristics of pathogens without having previously had contact with this pathogen. They can absorb (phagocytize) the pathogens and lock them inside - literally "eat" them, which is why they are also known as phagocytes . Then they present fragments of the pathogen on their surface to the cells of the adaptive immune defense (B and T lymphocytes), which then go into an activated state. Some immune cells can then kill the pathogens directly through phagocytosis or the release of aggressive substances, others begin with the production of antibodies that bind to the pathogens and on the one hand make them immobile and thus harmless, on the other hand mark them for destruction by other defense cells. After the first infection with a pathogen, the antibodies and so-called memory cells are retained in order to be able to react much faster and more efficiently to the intruder in the event of a renewed infection.
Whether an illness actually occurs after an infection depends on a complex interaction between the immune system and the (uninvited) guest. The amount of pathogens introduced and their pathogenic properties ( virulence ), as well as the state of the immune system of the person concerned, play a role . Previous contact with this pathogen can result in immunity , the dose or virulence of the pathogen may be too low for an outbreak of disease or the immune system may be able to prevent symptoms of disease despite infection [inapparent infection or silent celebration (immunization without vaccination or disease) ]. With an intact immune system and a low dose of pathogen, a disease such as a cold may either not break out at all or be less severe. As long as there are no clear symptoms, the course of an infection can hardly or not at all be predicted.
If a pathogen or a tumor cell does not generate an immune response, i.e. it escapes the immune system, this is known as an Immunescape .
Maturation and aging
A newborn's immune system is still immature. However, it receives maternal IgG antibodies via the placenta even before birth . In many mammals, antibodies cannot pass the placenta at all; they are then absorbed via the antibody-rich colostrum . The so-called nest protection helps the babies in the first few months. In addition, breastfeeding can protect against infections of the upper respiratory tract and gastrointestinal germs for a longer time through unspecific sIgAs that attach to the mucous membranes. Since the transplacental antibodies in the baby's blood are broken down with a half-life of about 3 weeks, an IgG deficiency in the serum occurs within 3 to 12 months after birth; as a result, the risk of infection increases. On the other hand, the own IgM level increases, which indicates that the adaptive immune system becomes active immediately after birth. The baby's IgG and IgA can only be clearly detected in the serum from about 6 months onwards and increase continuously until the values of adults are reached after several years. This initial delay is due to the immaturity of the T cells, so class switching is initially inefficient.
At birth, the child's immune system is predominantly set to anti-inflammation: no NK cells are detectable, there are immature B cells and there is increased T suppressor activity (due to the predominantly present T H 2 and regulatory T cells) . This is supposed to suppress a T H 1-dominated response in order to protect the body from rejection reactions. A change is therefore required for a successful defense against infectious agents. The colonization of microorganisms triggers signals for a change and trains the immune system. Also, live vaccines contribute by so-called heterologous effects on; these can lower child mortality more than expected and reduce the frequency of infections, including against non-vaccinated diseases.
However, recent studies show that the T cells of newborns can certainly react with an inflammatory reaction (pro-inflammation) and are not necessarily suppressed. T cells release interleukin-8 (IL-8), which u. a. neutrophils recruited. These then act unspecifically against harmful intruders. In contrast, T cells producing IL-8 are rarely present in adults, so that T cells in newborns differ qualitatively from those in adults.
In the first months of life, the immune system begins to prepare itself to defend itself against disease cells. This happens through a process of negative selection ; that is, the body initially forms many millions of different defense cells through random genetic recombination, each of which can recognize a different antigen. Then those cells are eliminated that would trigger an immune reaction to the body's own structures (this process is summarized under the term self-tolerance). In the case of T cells, this happens in the thymus, the place where the T cells mature. Here the T cells differentiate into the different types (such as CD4 + and CD8 + cells) and are then confronted with the body's own substances. When a T cell carries a suitable receptor and binds to the body's own structure, the T cell dies. The immune system learns to differentiate “foreign” from “own”.
The immune system reaches full functionality in adolescents. Statistically speaking, adolescents or adults are less ill than newborns / small children or older people because new T cells are constantly being formed in the thymus. However, T-cell formation gradually decreases in adulthood, parallel to the number of naive T-cells. The composition of the T cell pool therefore changes in an unfavorable direction.
With advancing age, around 60, people's susceptibility to diseases and other disorders increases again. This is mainly due to the fact that the formation of B and T lymphocytes decreases with age. This can be problematic in the case of influenza infections, for example, as fewer influenza- specific T cells and a smaller, effective amount of influenza hemagglutinin- specific antibodies (so-called lower antibody titers ) are formed, be it after a vaccination or after an infection. The immunosenescence concerns innates and adaptive immune system, but is not necessarily itself depends on age. Furthermore, the immune cells are less active overall, which leads to a weakening of the immune system, along with an increased risk of infection and cancer. In addition, the basic level of inflammation increases.
Disorders and diseases
As with all biological systems, errors can creep in with the immune system. The immune system can lose its ability to react appropriately to pathogens or the body's own cells: depending on the cause of the disorder, the immune response is either too weak or even absent or the immune response is too strong and excessive. The cells of the immune system can also become malignant and trigger cancer. An influence of depressive disorders, stress and other mental illnesses on the immune system is also suspected.
If individual components of the immune response are missing or if they no longer function properly, the immune system can no longer fight pathogens effectively and even diseases that are normally harmless can become life-threatening. Immunodeficiencies can be congenital or acquired:
- The severe combined immunodeficiency (SCID) is a group of congenital immunodeficiencies, which are characterized as well as the humoral immune response by affecting both the cellular immune system, therefore, "combined", the designation.
- The acquired immune deficiency AIDS is triggered by the HI virus , which successfully evades the immune defense by attacking the T helper cells. However, as the HIV virus multiplies, more and more immune cells are destroyed, so that after a few years of incubation, the immune system becomes increasingly weak and the number of infections and tumor diseases increases.
- A neutropenia or even agranulocytosis may by side effects of certain medications (eg. As cytostatics ) or are triggered by autoimmune diseases and mainly leads to mucosal inflammation and so-called opportunistic infections by otherwise harmless pathogens.
- Other congenital immune defects are: Behçet's disease , DiGeorge syndrome , selective immunoglobulin A deficiency and Wiskott-Aldrich syndrome , each of which has a certain part of the immune system disrupted.
Excessive immune response
- Autoimmune diseases : The protective mechanisms of self-tolerance do not always work properly, which can lead to dangerous autoimmune diseases in which the immune system attacks the body's own structures. In these diseases, the usually very well balanced balance between on the one hand the potentially self-destructive (autoreactive) T cells and on the other hand the regulatory T cells, which are actually supposed to "keep the former in check", is disturbed. Some examples of autoimmune diseases are:
- Cytokine storm : An overreaction of the immune system in which the usual regulation of the formation of cytokines does not work.
- Allergy / hay fever : The immune system can lose the ability to react appropriately to foreign proteins. The excessive activation of basophils (and eosinophils), but especially the local mast cells , can lead to allergic reactions such as hay fever. Systematic activation of these cells, i.e. activation throughout the body, can trigger severe symptoms up to anaphylactic shock .
The cells of the immune system can also degenerate malignantly and thus lead to cancer diseases that mostly affect the entire body and primarily take place in the organs of the immune system and lead to a decrease in the immune system and the suppression of normal blood formation in the bone marrow. Due to the large number of different cells and their precursors, there is a multitude of different cancers with very different symptoms and disease courses, which, however, can be roughly divided into two groups: If the cancer originates from the precursor cells in the bone marrow, one speaks of leukemia , which can be acute or chronic. Malignant tumors of the lymph nodes are called lymph node cancer or malignant lymphoma.
On the other hand, a therapeutic approach to cancer, cancer immunotherapy , is the activation of the immune system against tumor cells.
Other weak points
- If viruses have enveloped themselves in a layer that the body does not recognize as foreign (for example a layer of lipids ), they are not recognizable.
- In contrast to pathogens, tumor cells do not cause an inflammatory reaction, so there is no activation of the immune response. Some tumors have the property of literally camouflaging themselves. If no tumor-associated antigens (TAA) are produced by the cancer cells, the immune system will therefore not recognize the cancer cell and cancer growth and / or metastasis will occur .
- As far as we know today, the immune system does not protect against prions (infectious proteins), but appears - on the contrary - to play a role in the spread of prion disease. In one experiment, for example, mice with a defective immune system were immune to introduced prions, while animals with a functioning immune system developed a disease.
If the immune system is intact, one speaks of immune competence . The defense function can be positively or negatively influenced in various ways:
The idioms "strengthening the immune system" and "strengthening the immune system" are often used as a claim in advertising for dietary supplements, functional food and alternative medicine remedies. Therapeutic fasting also claims to strengthen the immune system. The problem here is a lack of medical definitions for what is meant by “strengthening”. Such references to general, non-specific benefits of a product are prohibited under EU law under Article 10 Paragraph 3 of the Health Claims Regulation , unless they are accompanied by a special health-related claim approved by the European Food Safety Authority. For inclusion in the corresponding positive list of approved information, the way in which the product affects the immune system must be stated and the effectiveness scientifically proven.
The basis for a healthy immune system is a balanced human diet , which contains all the substances necessary for the organism, such as minerals (especially iron , zinc and selenium ) and vitamins , and adequate sleep; Furthermore, long-lasting (chronic) stress should be avoided.
Methods from the fitness and wellness area are also advertised as measures to "increase immune function", which is then to be proven in individual cases with studies on special functions of the immune system. Examples of this are: regular hardening , for example by taking a sauna and using Kneipp baths . A meta-analysis shows that regular sport (compared to no sport) may reduce the duration and severity of respiratory infections, but not the incidence of the disease. According to such studies, psychotherapeutic procedures, in particular methods for coping with stress, are said to have an effect on the functions of the immune system. Clinical hypnotherapy has developed suggestive methods for treating individual immune diseases.
Nevertheless, there are no scientifically meaningful studies to what extent saunas, cold baths or cold showers, treading water according to Kneipp or individual foods have an influence on the immune system. If the immune system does not work from birth or due to illnesses (e.g. HIV) or due to malnutrition, there is no evidence of how it can be "strengthened" or how its performance can be improved. An increase in performance could even be harmful (allergies, asthma) or life-threatening (cytokine storm) (see also immune system # excessive immune response ).
Based on the recent discovery that the central nervous system of people, the vagus nerve with the immune system interacts , Dutch researchers were able to mid-2016 using tiny electrical impulses (similar to the effect of a pacemaker ) significantly positive effects in the treatment of autoimmune disease rheumatoid arthritis achieved.
In some situations, immunosuppression, i.e. drug inhibition or even complete suppression of the immune response, is necessary. This is the case, for example, with patients who have received a foreign organ as a transplant . Immunosuppression is also sometimes necessary for autoimmune diseases (including diseases of the rheumatic type ) and allergies. The longest known immunosuppressive drug is cortisone , the precursor of the body's own hormone cortisol . However, newer active ingredients such as tacrolimus or cyclosporin A are sometimes significantly more effective and / or have fewer side effects.
Vaccination is a method of strengthening the immune system and a preventive measure against certain infectious diseases . In the active immunization , the most common form of the vaccine, the immune system to form an immune competence is stimulated without the disease trigger itself. For this purpose, weakened pathogens, dead pathogens or certain typical proteins (proteins) and sugar molecules , i.e. fragments of the pathogen, are introduced into the body as vaccines . The reaction of the organism to these antigens leads to the formation of specific antibodies and memory cells, which continue to circulate in the blood and the lymphatic system, whereby the protection against these antigens is maintained for a long time. If the body comes into contact with the pathogen again, the memory cells provide it with a much more efficient and faster immune response that fights the pathogen before it becomes ill.
Aside from aging, there are other factors that can damage and reduce the function of the immune system. These include, among other things, severe health impairment due to previous damage such as in chronic illnesses, drug immunosuppression such as after organ transplants , drug abuse (including nicotine and alcohol ), malnutrition and the associated deficiency of vitamins and trace elements , an unhealthy or unbalanced diet, the absorption of environmental toxins from the environment, exposure to ionizing radiation , persistent stress , too little sleep, lack of exercise and also excessive exposure to cold in the sense of prolonged cooling or even hypothermia ( hypothermia ). In sport, after exhausting exertion, there is a temporary impairment of the immune system, which is known as the open window phenomenon . A combination of several factors can of course put increased stress on the immune system.
Psychological factors such as stress also affect the immune system. Stress leads to the shutdown of physiological processes in general that require a great deal of energy but are not necessary for short-term survival. This also includes the immune system. The immunosuppressive effect of stress is caused by the release of glucocorticoids (especially cortisol in humans) from the adrenal cortex , which in turn is triggered by adrenocorticotropin from the anterior lobe of the pituitary gland , which in turn inhibits the production of cytokines. In the case of chronic stress, the adaptive immune system is restricted, which performs its protective function via T and B cells.
Sunlight and vitamin D.
Sunlight can have an impact on the immune system. More than 100 years ago, daily sunbathing was an integral part of tuberculosis therapy (although in the absence of antibiotics , which were only discovered around 1930 and tested on humans after 1940). The research was able to show the underlying mechanism: Certain immune cells have a so-called toll-like receptor on their surface ; This is activated in the event of a bacterial infection and causes the immune cells to produce a precursor of vitamin D (25-hydroxy-vitamin D). At the same time, the same cell is increasingly developing another type of receptor that specializes in recognizing vitamin D. The sunlight converts the vitamin D precursor into the active vitamin D, which now attaches to the receptor. This stimulates the immune cells to produce the antibacterial cathelizidine . The connection also explains why people with dark skin are particularly susceptible to infections such as skin tuberculosis: As a rule, significantly lower amounts of the vitamin D precursor are found in their blood, although it is discussed whether the additional intake of vitamin D is being discussed -Preparations to strengthen the immune system can compensate for the deficiency.
For people with no or only moderate vitamin D deficiency, vitamin D supplementation is unlikely to protect against viral respiratory infections.
A weakening of the immune system can be a consequence of the immunosuppressive effect of UV-B rays , which disrupts the T-cell- dependent immune response. Excessive UVB exposure of the skin promotes the development of malignant skin tumors such as basal cell carcinoma and squamous cell carcinoma and significantly reduces the defense against pathogens such as bacteria, fungi or viruses. Diseases caused by parasites such as leishmaniasis , schistosomiasis or malaria are more severe and longer after UV exposure.
The complex interrelationship between the host organism and the pathogens can be viewed from an evolutionary point of view as an "attacker-defender system". The defensive measures of the immune system create a strong selection pressure , under the influence of which the pathogens have to adapt better and better to the (human) organism in order to continue to exist. At the same time, pathogens or parasites exert selection pressure on the host's immune system, which can lead to a coevolution of the parasite and host, which can lead to a symbiosis . Then the former pathogens can use the host for their reproduction without harming it. An example of such a successful coevolution are the mitochondria , which formerly invaded the cells of eukaryotes as an exogenous pest and which developed into an important cell organelle over the course of millions of years .
In the case of infections with pathogens that are adapted to humans as their reservoir host, an illness - with an intact immune system and a low dose of pathogen - can either not break out at all or take a less severe course. In the case of infections with pathogens that are not or only poorly adapted to humans, it depends on many factors (condition of the immune system, aggressiveness of the pathogens) how severe the disease is and how long it lasts or whether the sick person even dies as a result of the infection. According to this theory, the level of average mortality of a disease allows conclusions to be drawn, for example, about how well or poorly pathogens are adapted to humans.
With this evolutionary approach, many immunological processes can be better understood and interesting insights into the tribal history of the pathogen can be gained. In many scientific studies, indications for the correctness of this approach have been found, but there are still just as many contradicting results, so that this evolutionary theory of immunology cannot yet be conclusively assessed.
Influence on partner choice
Studies with animals at the Max Planck Institute for Immunobiology have shown, among other things, a connection between the individual immune system of a living being and the choice of partner. Genetic individuality and diversity can be recorded and evaluated via the sense of smell. The investigations showed: MHC peptides allow the immune system to obtain information about the status of individual cells by analyzing the MHC peptide complexes on the cell surface by the T cell receptors. And the analysis of the structure of these peptides enables information about the genetic status of a counterpart to be obtained via olfactory neurons. This is possible because the structure of the anchor residues of peptides allows conclusions to be drawn about the structure of MHC molecules and thus conclusions about the coding capacity of organisms.
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