Which one is beneficial, which one is harmful... How does the immune system know
Bacteria play a key role both in the early development and in the lifelong functioning of the immune system in the intestinal mucosa. By stimulating the lymphoid tissue located near the intestinal mucosa, they enable it to produce antibodies against pathogens.
The immune system leaves beneficial bacteria alone and fights against harmful ones.
As soon as a baby is born, bacteria colonize its digestive system. The first bacteria to settle influence the immune system's response, ensuring that they themselves are recognized as belonging to the host. Accordingly, these first bacteria determine the composition of the intestinal flora that will exist throughout a person's life. This is why the immune system of a baby born vaginally is stronger than that of a baby born by cesarean section.
Dr. Habil Jürgen Schulz of the Berliner Post has found that a baby's intestinal flora is shaped after birth according to whether they are fed breast milk, cow's milk, or formula.
Accordingly, while the pH value of the intestinal contents of breastfed children is between 3.5 and 5, in formula-fed children the pH value is 7 or slightly higher.
The digestive organs produce an average of 7-8 liters of secretions (enzymes, hormones, vitamins, acids, and alkaline substances) per day. The most ideal enzyme release occurs when the pH value is between 4.5 and 6.5.
The main factor that enables a healthy oral and intestinal flora to form in a baby is the initial flora swallowed from the mother's vagina during birth. However, once the baby is weaned, the bacterial mix in the intestines usually shifts from facultative aerobes to obligate anaerobes.
Some members of the intestinal flora, such as certain Bacteroides species, alter their surface antigens to resemble the host's cells, thereby escaping the immune response. Some harmful bacteria also use this strategy.
Bacteria affect a condition called oral tolerance — meaning they make people less sensitive to an antigen produced by bacteria taken orally or present in the digestive system.
A HEALTHY INTESTINAL FLORA PREVENTS ALLERGIES
It has also been observed that bacteria prevent allergies — that is, the immune system's overreaction to harmless antigens. When examining the flora of infants and young children, it has been found that those with allergies, or those who develop allergies later in life, are more likely to have harmful species such as Clostridium difficile and Staphylococcus aureus in their intestinal flora, while having lower numbers of Bacteroides and Bifidobacteria.
We can explain this observation as follows: since bacteria train the immune system during infancy, if these bacteria are lacking at the right time, the resulting under-trained immune system may overreact to antigens. However, a disturbance in the flora can also be a consequence of allergies, rather than their cause.