For decades, cases of allergies – spring, food, skin… – have skyrocketed, to the point of being considered a full-fledged global epidemic. This striking phenomenon raises many questions: what are the risk factors for allergies? Is it positive to be infected by pathogens to prevent them? Do the chemical agents to which we are increasingly exposed every day have an influence? Is genetics to blame and should we resign ourselves?
Recent studies add one more factor to the complicated equation: the microbiota. We will try to clarify all these issues.
The hygiene hypothesis
In 1989, epidemiologist David P. Strachan proposed the so-called “hygiene hypothesis” to explain the increase in the incidence of allergic diseases such as asthma, hay fever and eczema. After following 17,414 people until the age of 23, Strachan proposed that growing up in a family where several children live together decreases the likelihood of suffering from allergies. The reason? The increase in cross infections between siblings.
The idea then emerged that the reduction in infections among inhabitants of advanced Western societies due to hygiene and medical advances also led to a decrease in type 1 immune responses, which protect us against pathogenic viruses and bacteria. And this in turn carries a greater risk of uncontrolled type 2 immune responses; that is, allergic diseases. This last type of reaction protects us against helminth parasites (worms), physical and chemical damage, poisons, etc.
Later studies qualified this hypothesis by concluding that not all infections prevent allergies. The opposite may even occur: infections with respiratory syncytial virus or rhinovirus at an early age are related to a greater probability of suffering from asthma.
The importance of acquiring a healthy microbiome in childhood
Nowadays, it is considered that tolerance to allergens – that is, the absence of reactions of the immune system to elements that do not constitute a risk to our health – depends on appropriate microbial colonization and an immunostimulatory environment from the first moments of life. And both are, in part, acquired from the mother.
In this context, the epithelia (skin, intestine, respiratory tract, etc.) receive the appropriate stimuli (although chemical damage can disrupt this process) so as not to develop an adaptive immune response to allergens. On the one hand, this contributes to tolerance, and on the other, to maintaining the physical integrity of said epithelia, which prevents a harmful inflammatory response and increased risk of infections.
In fact, it has been described that diaper rash lasting more than a month in babies is related to food allergies. As a result of this finding, the proposition has arisen that the first exposure to foods that trigger frequent allergic reactions, such as peanuts, is through ingestion. This would induce tolerance, as if telling the immune system: “this is a food, not a pathogen.” On the other hand, the use of moisturizing creams with components of food origin, such as peanut oil, would promote not only skin irritation, but also an allergic reaction.
Bacteria related to asthma
If the homeostatic (balance) situation between the immune system and the microbiota does not occur in the first years of life, we could then face dysbiosis or pathogenic microbiota. To delve deeper into this connection, the risk of suffering from asthma and its relationship with the microbiome has recently been studied.
Specifically, the researchers examined the set of genes of bacteria present in the feces of pregnant women in their third trimester and in children between three months and one year of age. Well, the authors of the study found certain groups of bacteria that do differ between children with and without asthma. In addition, they determined that cesarean delivery reduces the presence of the Bacteroides group, which is linked to greater probabilities of suffering from the disease among young children.
Mice cured with bacteriotherapy
In the case of food allergies, healthy microbiota has also been shown to induce tolerance to allergens, largely by promoting the formation of regulatory T lymphocytes (immune response inhibitors). A study conducted at Boston Children's Hospital, led by doctors Rima Rachid and Talal Chatila, found dysbiosis in children with food allergies.
In addition, the scientists administered bacteria (bacteriotherapy) from the Clostridiales and Bacteroidales groups to mice susceptible to allergy due to a genetic change in the interleukin-4 receptor (a protein that plays very important roles in type 2 immune responses) and They observed that they improved. That is, the microbiota is very relevant in the development of allergies, but we must not forget the genetic component.
In this sense, certain genetic mutations illustrate that allergies are the product of “exacerbated and out of context” type 2 immune responses. These mutations, which overactivate the transcription factor STAT-6, produce very serious allergic diseases characterized by atopic dermatitis, elevated immunoglobulin E (IgE) in serum and a high number of eosinophils (a type of immune system cells), along with another type of development problems.
In conclusion, knowing the genetics underlying allergies will allow the development of new drugs, such as interleukin-4 receptor blockers. Furthermore, elucidating how the “healthy” microbiota is capable of preventing these disproportionate responses of the immune system can lead to the creation of probiotic treatments that reduce them and prevent the most dangerous anaphylactic reactions.
This article has been published in «The Conversation«.
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