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Defense against infections

Last update 1-11-2008

The developments around birth

This page presents the development of the defense against infections around birth. This is a special period where the child is first confronted with the microorganisms in the world outside the womb and has to learn to co-exist with them. The way this happens has significant influence on the child's future health.

Before birth

Before birth the unborn child is protected by its still primitive defense system and mama's immune system. As the fetus develops its innate defense and ideotypic immune system provide a means of protection in case bacteria manage to penetrate into the child's bloodstream. Its adaptive immune system is developing but has no experience yet and it therefore not functioning.

During birth

During birth two important things happen:

  1. The child ceases to be linked to mama's bloodstream so her immune system can no longer protect it
  2. The child is confronted with the outside world with all its microorganisms and toxins. The moment it leaves the birth channel the child's skin and the gastro-intestinal tract get colonized by microorganisms, mainly inherited from its mother and any other people who handle the child shortly after birth.

This means that the passively obtained antibodies from the mother and the child's ideotypic and innate immune systems provide its means of defense, i.e. the only protection against infections at this point in time. As its digestive tract is not yet colonized, the colonization resistance provided innate defensins and a selected microorganisms has not developed yet so cannot offer protection.

The first weeks after birth

During the first weeks after birth several things happen, assuming no contamination with major infectious microbes occur and the child is being breast fed.

During this period when the child's defenses against infections are still immature the mother's defenses aid the child by antibodies (immunoglobulins) transferred to it with the milk it is being given by mama. More on the effects on breast feeding is presented below.

Microorganisms colonize the digestive tract

As soon as the child is born, microorganisms will colonize its gut. The child will acquire these microorganisms from its direct surroundings, mainly from people with who it comes into close contact. As there are only a few if any competitors in the digestive tract in this phase, microorganisms will multiply rapidly. To control this process the child's gut epithelium produces defensins which dampen the growth of microorganism which are susceptible. Soon the microorganisms which are resistant and survive, will also produce defensins to limit competition and the full chemical warfare between microorganisms has started.

The adaptive immune system is trained and develops tolerance

For the purpose of explaining this aspect of the development of the defence against infections, the microorganisms which colonize the child's digestive tract are divided in three groups:

  1. Microorganisms that do not translocate the gut wall (are not able to)
  2. Microorganisms that translocate the gut wall in limited numbers
  3. Microorganisms that massively translocate the gut wall

It is important to note that the number of microorganisms translocating the gut wall is directly related to their ability to do so AND to their numbers in the gut lumen.
The first group will not come into contact with the adaptive immune system and therefore does not add to its antigen-training.
The third group, the potential pathogen microorganisms, trigger the adaptive immune system to learn they are unwanted and harsh action is required.

Some microorganisms settle permanently and resist colonization by others

The second group is the most interesting one as, during the initial months of the child's life, they trigger tolerance in the adaptive immune system. T-suppressor cells are formed that prevent the adaptive immune system to react if one of these microorganisms translocates. These microorganisms are accepted as 'self' by the adaptive immune system and take permanent residence in the gut lumen.

After 4 to 6 months

In man, after about 4 to 6 months the adaptive immune system decreases to develop tolerance to microorganisms. Less new T-suppressor cells are formed. The microorganisms that have become tolerated by the adaptive immune system will remain tolerated. These microorganisms are both able to and are allowed to persist in the digestive tract lumen and thereby form the defence layer called the colonization resistance.
This may also account for allergens; if they reach the newborn at the right time and place early enough in life, tolerance may develop and therewith no allergic reaction to the allergen in question.

The importance of getting contaminated and colonized with intestinal bacteria after birth

In the figure below the development of microorganisms in the digestive tract of mice when entering the conventional world coming from a sterile environment is shown:

  1. Newborn mice which come from a sterile uterus environment
  2. Adult ex-germ free mice taken out a germ free isolator into the outside mouse world provided by caching them in a mousery

In the figure, on the left newborn mice are shown which are being breast fed. On the right colonization of adult Germ Free mice is shown which are taken out of their germ free environment. Both are confronted with the colonization of their digestive tract. In the newborn the same environmental challenge cause a gradual colonization pattern whereas in the adult ex-germ free animals an explosive colonization to significantly higher concentrations for a long period. In the first group no signs of infection where found at autopsy at day 10, while in the ex-germ free group spleen and liver cultures were positive, suggesting that in this group translocation from the gut had occurred.

The effects of breast feeding on the colonization of the digestive tract

The effects of breast feeding on the way the child's microflora develops

Breast feeding, as opposed to bottle feeding, has a significant effect on the colonization of the digestive tract by microorganisms. Human milk contains immunoglobulins (IgA) that are specific to certain microorganism the adaptive immune system of mother (or the female performing the breast feeding) has learned to be harmful. This helps the child's immune system to react faster, thereby limiting the numbers these microorganisms can grow to in its digestive tract. As the number of each microorganism is directly related to the number of that microorganism species that translocate, this has a significant effect on the child's wellbeing.