The importance of handwashing wasn't so self-evident over two hundred years ago. Heck, we could argue that some people still don't consider handwashing very important if the percentages of people not washing their hands after going to the bathroom are to be believed. It wasn't until the 19th century when people started seriously considering the implications of those microbes (however cute they may be) wriggling around on a microscope slide. Now, due to the influences of Oliver Wendell-Holmes, Ignaz Philipp Semmelweis, Louis Pasteur, and through vaccinations and antibiotics, improved food production, preparation and storage, and proper water management, we've seen major improvements, not only in quality of life, but the decline of morbidity and mortality.
But it isn't all sweet-smelling roses. The upward trend for chronic ailments is alarming and this is not just because people are living longer and significant proportions of the population is moving into the upper age brackets. There's evidence that urbanization may not solve all the problems--especially in children, developing asthma or allergies is increasingly likely. In 1828, John Bostock first noticed the curious correlation of allergies and the urban population (whereas the farming community, although exposed more frequently to pollen and other allergens were less affected). The frequency of these problems, however, seems to be restricted to first world countries in high socio-economic classes.
In 1989, David Strachan observed that sensitivity to allergens by a skin prick test was inversely proportional to the size of a family. He proposed that higher standards of personal cleanliness would reduce the chance of cross-infection in families and stunt the development of the immune system during childhood for kids who had not acquired infection from contact with siblings. Thus the "hygiene hypothesis" was born, that our current obsession with the elimination of germs has deprived our immune systems from seeing the microbes and other supposedly dangerous environmental stimuli helping our natural defenses mature.
In a recent issue of Cell, King et al. provided evidence for this hypothesis through a mouse model. The nonobese diabetic (NOD) mouse is a standard model of studying autoimmune diabetes--in this strain of mouse, insulin-producing cells are attacked by the immune system leading to the disease. So what exactly is happening?
Typical of a lot of laboratory mice, the NOD mice were raised in a sterile animal facility--an equivalent to our modern first-world, clean living spaces. These mice did not see many microbes and their immune system--particularly the memory T cells which help remember what foreign microbes and particles that have been encountered before--failed to mature properly. Without a population of memory cells, the CD4+ CD8+ T cells which can be skewed towards autoimmune activity expanded into this empty niche. What makes NOD mice particularly susceptible to the increased production is this strain's increased expression of IL-21 receptor. IL-21 is part of a vicious mechanism which helps drive the expansion of autoimmune reactive T cells.
Another group of NOD mice were inoculated with bacteria or bacterial products and showed a reduced instance of diabetes. The immune system of these mice were stimulated, boosting the population of memory cells and eliminating the extra niche for T cell expansion. So what does that mean for us? Children living on farms or had pets are far less susceptible to allergies to those that don't. Military cadets who have never been exposed to hepatitis A or other bacteria were more likely to have asthma. Should we all get inoculated with environmental bacterial endotoxins at an early age? Should every family be required to own a cat or dog to prevent the onset of allergies and other autoimmune diseases?
But don't throw out your bath towel just yet. Although the Cell paper may present some convincing data, remember that these experiments were done in a controlled mouse model. This may help those people with the human equivalent of autoimmune diabetes, but it doesn't explain the variety of other chronic diseases we experience. There could be a completely different explanation--the presence or absence of microorganisms is not the only difference between urban and country, first-world and third-world. Could it be the result of pollution or change in diet? A different lifestyle that has nothing to do with how many times you wipe down the countertop?
But whatever the case, I can very well picture this as a reason Junior might petition his mom to get his very own Fifi.
