We hear a lot about allergies these days. Asthma in children, which is caused by allergies, is said to be increasing, too. Allergies have a history and background and can be somewhat complicated to understand. Here, I would like to explain the concept behind allergies and why they are on the rise today.
In thinking about all childhood diseases, we should start with the idea of immunity. This is because until at least several decades ago in Japan, nearly all childhood diseases were infectious diseases caused by bacteria (pathogenic microorganisms or viruses). There are, of course, also various congenital deformities and diseases, and unfortunately, children with these conditions are more likely to contract infectious diseases, and in the past, most lost their lives to pneumonia or diarrhea before reaching adulthood.
It is well known that even healthy children contract all sorts of infectious diseases. Besides colds, children catch the usual childhood illnesses such the measles, rubella, chickenpox, the mumps, and whooping cough. This is because children born into this world live in a veritable sea of bacteria.
Even so, most children manage to survive because they have built up the defense mechanism of immunity. A newborn is protected by the immunity that is passed from the mother to the fetus through the placenta. This immunity is weak, and becomes strengthened in the process of repeated exposure to pathogens.
Progress in science and technology, in particular, medicine, has been responsible for advances in the treatment of infectious disease with the discovery of antibiotics, for example, but it has also improved public health and hygiene through better sewage systems so that we do not become ill from all bacteria around us. As a consequence, the chances of catching infectious diseases have also declined drastically, but on the other hand, we now see a wider range of diseases.
It is the defense mechanism of immunity that has enabled human beings to survive throughout history--this is what we have learned from experience. The most compelling case is an illness like the measles, which results in life-long immunity. In other words, once contracted, it is not contracted a second time. Attempts to scientifically elucidate life-long immunity and the causes of disease led to the systemization of bacteriology and virology and the discovery and classification of various pathogenic bacteria, microorganisms, and viruses. In parallel with these developments, immunity, which protects the organism, became conceptually systemized into the study of immunology. There were events that take place from the latter half of the nineteenth to the twentieth century.
The Chinese were the first to use life-long immunity to fight infection. In ancient times, smallpox was the most dreaded infectious disease, but it was also known that it produced life-long immunity. The Chinese took the pus of a person with smallpox and began inoculating healthy people with it. The technique of smallpox inoculation spread to Constantinople via the Silk Road where the wife of the British ambassador to the Ottoman Empire saw it being practiced and introduced it to England.
Based on this news and stories that contracting cowpox, a disease similar to smallpox in cows, protected humans from smallpox, Edward Jenner, a British scientist, attempted human inoculation with cowpox. The results of the cowpox vaccination showed that it was much safer than variolation, or deliberate infection with smallpox. Cowpox vaccination spread throughout Europe and reached Japan in the late Edo period or mid nineteenth century. Today, smallpox has been eradicated and smallpox vaccinations are no longer carried out, but this marked the beginning of the development of disease prevention with vaccines.
Building on the notion of life-long immunity, the discovery that vaccines such as cowpox vaccination could prevent disease suggested that humans could conquer all infectious diseases. With the discovery of pathogenic microorganisms and advances in immunology in the latter half of the nineteenth century, however, it became clear that things were not so simple.
Once pathogenic microorganisms were discovered and the causes of disease became known, it was found that animals and humans with life-long immunity carry proteins called in their blood that kill, congeal, or bind and agglutinate these microorganisms. Called antibodies or immunosera, they are classified into bactericidins or agglutinins. (Serum refers to the liquid component of the blood minus the cellular components.) Once a disease has been contracted, the body produces these substances to protect itself in reaction to bacteria.
In the late nineteenth century, it was found that when animals with life-long immunity were given antigens, substances which induce the immune response, they sometimes experienced respiratory difficulties or shock. In these cases, the protective immunity had the reverse effect and caused this state, which we call an "allergy." Literally meaning "other reaction," an allergy can be seen as an immune response that causes damage to a specific part or the entire body.
It gets quite complicated from here, but let me say that the lymphocytes and other cells in the lymphatic system and bone marrow, which we call immune cells, play a major role in the immune and allergic response. These immune cells as well as the cells mentioned above secrete antibodies (proteins) and cytokines that are involved in the immune and allergic response.
Although this is not the case in developing countries, since the mid-twentieth century, it has become extremely rare for children to die of infectious diseases in advanced countries, including Japan, where accidents and childhood cancer are the major causes of death. On the other hand, many children in these countries suffer diseases due to allergies. What then is the reason?
Immunity, which protects the body, and allergies, which induce illness, are responses by the same immune cells, and in this respect, we can think of them as striking a balance. According to an older colleague of mine who is an asthma specialist, since the decline of infectious diseases resulting not only from bacteria but also from roundworm and other parasites, asthma has increased. As society has grown more affluent and science has advanced, fewer children are dying of infectious diseases, but at the same time, more have diseases due to allergies.
One recent explanation for the rise in allergies is as follows. The immune cells in our body have two types of lymphocytes: Th1, which plays a role in infectious immunity, and Th2, which is involved in allergic response. When Th1 cells weaken, Th2 cells strengthen. In other words, a disruption in the balance between the two types of cells facilitates an allergic response. It has also been established that smoking and environmental hormones, dioxin, for example, can increase the strength of Th2 cells. This should serve as a word of caution for advanced society.
The idea that the increase in patients with allergies can be attributed to the balance between Th1 and Th2 cells is known as the hygiene hypothesis. Proposed by a British researcher nearly twenty years ago, simply stated, it posits that as societies become affluent, children are raised in hygienic environments, and as a result, opportunities for infection decline, infectious immunity weakens, and the allergic response strengthens.
We could say that raising children requires not only strengthening their minds and bodies, but also building up the immune system to protect the body against bacteria. In that case, they may need to play in the mud a little, too.
