The “hygienic theory” of disease says that children who are raised in very clean environments can suffer from allergic diseases and even childhood cancer. Apparently, the lack of “training” of the children’s defense system would lead to the development of these diseases.

In 1989 Professor David Strachan of the University of London published an interesting short study in which he found that younger siblings in a large family suffered less from allergies than older siblings. Dr. Strachan then postulated that by suffering more infections of the older siblings, the younger ones developed a more “trained” defense system and therefore developed fewer allergies.

On the contrary, the older siblings, growing up in a “slightly cleaner” environment, developed a defense system that was more likely to confuse pollen or cat hair with “more dangerous enemies” and developed more cases of allergies.

Since then, the term “hygiene theory” has been used to try to explain the increase in various childhood illnesses related to alterations in the defence system that are supposed to occur due to a lack of “training” of defence cells. Examples of these diseases include asthma, allergic rhinitis, childhood leukemias, multiple sclerosis, ulcerative colitis and Crohn’s disease.

A very interesting case concerns the development of B-type acute lymphocytic leukaemia (ALL), the most common childhood cancer. It turns out that it has long been observed that children who come to nests or nurseries have less tendency to get acute lymphocytic leukemia. It is believed, according to the “hygienic theory”, that the exposure of children to microbes from peers, makes their defense system grow “more trained and healthy”, something that obviously does not happen in the child who grows isolated and “very clean” at home.

Acute lymphocytic leukemia (ALL) type B is a consequence of the mutation and abnormal proliferation of a type of white blood cell called B lymphocyte and a recent American study describes in detail the mutations that occur in these cells in the development of the disease and how these changes, they give the reason that the “hygiene theory” would have much to do with this type of leukemia.

The B-type lymphocyte is a kind of watchman of the defense system and during its constant “patrolling”, the B-lymphocyte is attentive to the presence of viruses and bacteria in the blood in order to recognize them and eliminate them by producing special “bullets” called antibodies, which are very specific against these microbes. In other words, B-lymphocytes are super-specialized cells that are constantly adapting to the type of enemy in front of them, so each B-lymphocyte is different from the other.

But “something” happens during the development of B-type acute lymphocytic leukemia, which causes that instead of having multiple types of normal B-lymphocytes, only one type is produced, which multiplies disorderly and becomes a cancer cell, which invades the normal tissues of the child’s body.

According to researchers, “that something” is a disorder of the genetic system that allows B lymphocytes to differentiate from each other. When that genetic disorder occurs, all the lymphocytes that are produced are the same. It is as if the “photocopying machine” had broken down and that in spite of wanting to photocopy different documents, all the photostatic copies produced by the machine are always the same.

The genetic system that allows B lymphocytes to be different from each other depends on the action of two proteins, AGR and AID. The RAG protein allows the “rearrangement” of large portions of lymphocyte DNA, as if preparing them to recognize enemies, while the AID protein, which always acts after the RAG protein, “tunes” the lymphocyte DNA so that it learns to recognize the details of the enemy. In this process, the sequence of action is important, first the RAG protein has to act and then the AID protein.

What the researchers demonstrated in sophisticated experiments in mice is that when lymphocytes are exposed to early infections, that sequence is always respected and diverse B cells are obtained, while when lymphocytes did not have the “experience” of early infections, the two proteins, RAG and AID acted at the same time, causing mutations to occur that produced a single type of cancerous B cell.

The authors conclude that early exposure of B lymphocytes to various types of microbes makes the system efficient and prevents leukemia.

But what about the “hygiene theory”, does it make sense?

There is no doubt that, together with the advent of antibiotics, the hygiene of the individual and the environment constitutes one of the great achievements of public health in the last century, but will it not be possible that “we have gone too far” and that we have become too clean and that this excess of zeal in cleaning is causing our children to grow up with weaker defense systems and therefore more susceptible to various diseases, including allergies and cancer?

A very interesting case, and which is used to reinforce the validity of that theory, is that of the frequency of allergic diseases in the two Germanies, before unification in 1989. Before that date, allergies were much more frequent in West Germany, much more urbanized, than in East Germany, much more rural.

The fact that the medical systems of the two Germanies were very similar eliminated the possibility that allergy diagnosis was more efficient in West Germany and it was therefore thought that allergies were less frequent in East Germany because children grew up more in contact with animals and nature. The fact is that after unification, when East Germany began to urbanize rapidly, allergy rates have equalized in that country’s western and eastern regions.

In the case of leukemias, Dr. Mel Greaves of the Cancer Research Institute in Surrey, England, says in the book “Hygiene Theory and Darwinian Medicine” that this disease would be more frequent in children who grow up in very clean environments for two reasons, the first is that when they acquire common infections later in life, the immature defense system “gets disoriented” and reacts wrongly to the infection, and the second is due to a certain genetic susceptibility that makes their defense system not respond adequately to infections.

In addition to the study described in this article, a recent review of the subject by researchers at the Massachusetts Institute of Technology and Harvard University concludes that it is possible that the “hygienic theory” has validity by impeding the proper development of the defense system.

What to do, how to raise children?

I think the first thing is to say that cleanliness is necessary, but it seems that over-cleaning is not. In that sense, allowing the child to have contact with nature, to play with dirt in the park, to come into direct contact with pets, to suddenly eat a piece of bread that fell to the ground should not be considered capital sins.

It is also important that you stop using soaps and detergents that have “antibacterial substances” and that you are not fooled into saying that “maintaining a disinfected environment” is good for your health. These antibacterial soaps and detergents not only contribute to the development of antibiotic-resistant bacteria, but may also cause illness through “excessive disinfection” or a direct effect.

Triclosan, a commonly used antibacterial, has been linked to the development of liver cancer. Childhood leukemia is a tragedy when it occurs in a family, who knows if letting our children grow up in greater contact with their environment can decrease the risk of developing that disease.