Defense mechanisms - General infection theory

The human body can defend itself against invading pathogens with physiological defense mechanisms. Some people 'derail' these normal defense methods and there are pathological defense mechanisms.

Defense mechanisms - General infection theory
The human body can defend itself against invading pathogens with physiological defense mechanisms. Some people 'derail' these normal defense methods and there are pathological defense mechanisms.

Defense mechanisms


The human body can defend itself against invading pathogens with physiological defense mechanisms. Some people 'derail' these normal defense methods and there are pathological defense mechanisms.


Physiological defense mechanisms

Physiological defense mechanisms include the following protection methods:

  • Natural barriers;
  • Non-specific defense;
  • Specific defense.


Natural barriers

Natural barriers are formed by undamaged skin and mucous membranes that separate our bodies from the outside world.

  • The skin acts as a mechanical barrier and thus prevents micro-organisms from entering the body. Moreover, healthy skin contains a layer of skin fat that has a bactericidal (bactericidal) effect.
  • Mucous membranes are covered with secretions that combat microorganisms. For example, the mucous membranes of the eyes are covered with a layer of tear fluid that contains the enzyme lysozyme. This enzyme kills bacteria. This enzyme is also found in saliva.


The mucous membrane of the airways fights the bacteria in a different way. In the serpentine passages through the nose a large part of the dust remains suspended from the inhaled air. When micro-organisms reach the lower airways, they are transported upwards by the coordinated stroke movement of very small, mucus-covered cilia. The microorganisms are further removed by coughing.


The mucous membrane of the mouth and the gastrointestinal tract also has a transport mechanism. Moreover, the defense function is supported there by saliva (causes dilution), stomach acid, and by the antibacterial activity of pancreatic enzymes, bile and intestinal juices.


The bacteria that live in the intestines under normal conditions (the intestinal flora) prevent the colonization of pathogenic bacterial strains. This so-called colonization resistance can be disrupted by the use of antibiotics, because the protective bacteria are then killed. Such a disturbance can lead to overgrowth with pathogenic intestinal bacteria, resulting in a clinical intestinal infection with diarrhea.


Non-specific defense

Non-specific defense is a term that indicates that this defense mechanism always attacks the attack, regardless of the nature of the invader (microorganism, chemical, heat). A general term for the (foreign) invaders is: antigen.


This non-specific defense is based on two mechanisms:

  1. Cellular defense: based on cell responses;


In case of tissue damage this defense mechanism ensures that the blood supply to the damaged area increases. White blood cells can more easily leave the blood vessels and move to the relevant area. In addition, the bone marrow releases large numbers of white blood cells from stock and starts the production of new white blood cells. The neutrophil granulocytes are the first of the white blood cells. They can 'eat up' invading organisms (phagocytose) and thus try to delineate the infection to a small area. Secondly, the monocytes come into action. These cells have a larger phagocytic capacity.

      2. Humoral defense: based on chemical substances that are released.


The first group of antibodies that come into action with this mechanism consists of certain defense proteins (antibodies). They can bind to the intruder and form a so-called complement system. This is easily recognized and phagocytosed by the white blood cells. The second group of antibodies is formed by the cytokines. These are substances that act on the cells themselves, on neighboring cells or on remote cells. The cytokines thus warn other cells to take action as soon as an inflammatory process is under way. For example, fever can occur. The increased body temperature stimulates the defense mechanisms. With relatively little discomfort for the patient, fever thus makes an important contribution to the destruction of antigens.


Specific defense (immunity)

This type of immune response is capable of producing substances that are exclusively directed against a specific invader. In the development of this powerful immune response, three phases can be distinguished:

  1. Presentation phase: during the penetration of the micro-organism (antigen) it becomes clear which antigen is involved; the antigen 'makes itself known';
  2. Activation phase: cells are now being made against the antigen. On the cell wall of the immune cells are structures that 'fit like a key on a lock' onto the antigen;
  3. Attack phase: it takes about seven days to create enough cells. They are then present in sufficient quantities so that the antigen can be actively controlled. After healing, the information about the antigen remains in certain white blood cells. These are memory cells. With a subsequent contact with the same pathogen, the body already has a number of suitable immune cells. The counter-attack can therefore be deployed almost immediately and the production of new specific immune cells now only takes two days. The memory cells remain in the body for many years and sometimes for life. This system of recognition of pathogens is called immunity.


There are four ways that someone can become immune:

  1. Natural active immunization: going through a disease;
  2. Artificial active immunization (vaccination): conscious administration of weakened germs. Vaccination allows patients to become insensitive to certain illnesses without actually experiencing these diseases. By introducing a small amount of the (dead or weakened) pathogenic microorganism into the body, the body starts with the production of memory cells. Because so few germs have entered, the body can switch them off in time and there is no disease (sometimes some mild symptoms). When the body naturally comes into contact with the microorganism in question, it is in fact already prepared for the attack and the multiplication of the pathogen can be stopped in time. This form of immunity is known as artificial-active: the microorganisms are initially artificially administered, but then the body has actively formed defense mechanisms against them;
  3. Passive artificial immunization: the delivery of ready-to-use antibodies against a specific micro-organism. An example of this is the administration of antibodies (immunoglobulins) against hepatitis A to travelers to tropical regions;
  4. Passive natural immunization: the transfer of immune cells from the mother to the unborn child. After birth, the child will have access to these immune cells for a few months, which he received during pregnancy.    


Pathological defense mechanisms   

Pathological defense mechanisms include allergy, immunodeficiency and autoimmune diseases. Below is an explanation of each of these disturbed reactions.



When the immune system becomes over-stimulated, a harmful situation can arise because the body reacts excessively to a certain antigen. This is called allergy or hypersensitivity. The body has already been in contact with the relevant antigen or with 'family' of the antigen at an earlier time. As a result, hypersensitivity has developed at an earlier stage and this is known as sensitization. Frequent contact with certain antigens may increase the likelihood of sensitization. Such an allergic reaction can occur in four different ways and is indicated by type I, type II, type III or type IV hypersensitivity reaction.               


Allergic reactions often arise on contact with substances that have nothing to do with microorganisms, such as cat hair, house dust mite or pollen. Such substances are called allergens because they are capable of inducing an allergy. In the case of allergy to a specific substance, there is usually a hereditary predisposition. One then has a so-called atopic constitution or allergic constitution. People with an atopic constitution have a high chance of developing allergies to a specific substance.   


In some infectious diseases, allergic reactions can be caused by (temporary) derailment of the immune system. Typical of Pfeiffer's disease is that the patients often have allergic reactions to antibiotics during their illness.


Elaboration of the four different hypersensitivity reactions    

Type I. People with this type of allergy have an excess of antibodies (defense proteins) from a certain class, the immunoglobulin E (IgE). These immunoglobulins are bound to so-called mast cells in the tissues. As soon as an allergen passes by, it is bound to the IgE mast cell complex. This activates the mast cells and secretes histamine. This substance causes dilation of the blood vessels (swelling of the tissues) and narrowing of the airways. These two factors together lead to acute anxiety that can take on life-threatening forms. A wasp sting or a specific drug can give rise to a type I hypersensitivity reaction, but latex can also evoke such a reaction!


Type II. Certain antigens are known to bind to certain body cells. For example, some medicines bind to red blood cells. The body makes antibodies, as usual through the non-specific immune system, but not only against the antigen but against the whole complex of antigen and blood cell together. Thus, the antibody will destroy not only the antigen but also the blood cells.


Type III. Immune complexes play a role in this reaction. Immune complexes are antibodies with the antigens bound thereto. They circulate in the bloodstream and can deposit on the walls of blood vessels throughout the body. This causes tissue damage in all sorts of random places in the body.


Type IV. This form of hypersensitivity is also called contact allergy and is an excessive reaction of the memory cells. Upon a subsequent contact with the antigen, this delayed type of hypersensitivity reaction starts, which can cause tissue damage in the skin. The reaction often manifests itself in the form of eczema.



With a normal supply of germs, the immune response of the body can fail in certain circumstances. We call this an immunodeficiency. For someone with an immunodeficiency, the risk of developing an inflammation is increased. Both infections from the outside in the form of infectious diseases and the 'opportunistic infections coming from inside' are then threatening. The fungus Candida albicans is such a wolf in sheep's clothing, which does not cause any abnormalities in healthy people, but emerges as a formidable pathogen when the resistance of the host diminishes. These vulnerable patients should be treated with the necessary precautions, since not only the risk of developing an inflammation or infectious disease is increased, but the healing process is often delayed.


There are several factors that can lead to an immunodeficiency:

A. Reduced organ function based on:

  • Old age. This is a physiological change. That is why the annual "flu shot" is recommended for all elderly people.
  • Reduced movement of cilia on the mucous membranes (eg by smoking). The cleaning of the airways is then no longer optimal, increasing the risk of infections.


B. Reduced resistance based on:

  • Food deficiency due to long-term diarrhea, for example. This can lead to an increased susceptibility to infectious diseases.
  • Stress. The mechanism of action can not yet be clearly defined. However, a fact is that long-term exposure to physical or mental stress can drastically reduce resistance to infections.
  • Systemic diseases (also called underlying suffering). Examples of such diseases are:

- Diabetes: due to the vascular abnormalities that occur with this disease, the patients are less well equipped for the control of wound infections;

- Leukemia: a malignant (malignant) growth of white blood cells. The amount of white blood cells increases strongly, but due to changes in the cells, the immune function is unfortunately completely disabled. Leukemia patients are extremely susceptible to infections. If these patients develop a fever without other symptoms, it often appears that an existing gingivitis or periodontal disease is the source of infection. A healthy mouth is therefore very important!

- Aids: the immune system is strongly affected, increasing the susceptibility to infectious diseases and the development of opportunistic infections.


C. iatrogenic causes;


The word iatrogen is the scientific term for 'caused by medical action'. Although it is not a natural cause of immunodeficiency in this list, they are mentioned here because of the extra (!) Importance of good infection prevention in such situations:

  • Radiotherapy. Salivary glands may function less well as a result of irradiation. A dry mouth increases the risk of mucosal infections and caries.
  • Artificial valves, artificial hips and so on. The surface of these artificial structures is susceptible to adhesion and colonization of microorganisms. The risk of infections in these patients was previously labeled as increased. According to contemporary insight, no such antibiotics require antibiotic prophylaxis for bloody procedures.
  • Medication use. Four examples are 1 cytostatics, which are used in tumors. They virtually eliminate the entire immune system. 2 Immunosuppressants, which suppress the immune system to prevent a transplanted organ from being rejected. 3 Anti-inflammatories, these are consciously administered to reduce the natural defenses, soothing the pain. 4 antibiotics, these disturb the normal balance in the intestinal flora. For example, a fungal infection can occur after antibiotic use.


Autoimmune diseases

In all autoimmune diseases, body's own cells are destroyed by the body itself. This can sometimes result in an increased risk of developing inflammation or infectious diseases. In Sjögren's disease, for example, the production of saliva and tears was reduced. The mucous membranes of the eyes and mouth are therefore very dry, which may cause infections. The risk of caries developing is also greatly increased in these patients.