The good side of a viral infection?

A gene linked to asthma and allergies also has a role in hepatitis A viral infections, according to a new study. This unexpected discovery has investigators speculating that a past infection with the hepatitis A virus may protect some people from asthma and allergies.

There's been a dramatic rise in the number of people with these conditions over the past few decades, and some scientists blame it on modern society's cleanliness. In a theory labeled the hygiene hypothesis, they argue that our immune systems are no longer exposed to enough microbes early in life. This sets the stage for the body's defenses to overreact to normally harmless stimuli, causing allergies and asthma (SN: 8/14/99, p. 108).

Dale T. Umetsu of Stanford University and his colleagues now propose that mild hepatitis A infections may ward off such immune dysfunction. In the Oct. 9 Nature, they report that a gene called TIM-1 governs the development of allergies and asthma in mice. Other research had shown that the hepatitis A virus enters cells by attaching to this gene's protein. Umetsu's group further found that among people infected with the virus, those a certain version of TIM-1 had a lower incidence of allergies and asthma than did people without this variant.

Before 1970, almost everyone in the Western Hemisphere typically became infected wire hepatitis A. Today, less than one-third of people in those countries contract the virus. Umetsu and his colleagues plan to investigate whether hepatitis A vaccinations, which aren't routinely given in the United States, have antiasthma and antiallergy effects.--J.T.