Hepatitis A May Prevent Allergies in Some
Common gene variation combines with virus to bolster immune system
WEDNESDAY, Oct. 8, 2003 (HealthDayNews) -- Infection with the hepatitis A virus may provide some protection against asthma and allergies, but only in people with a common form of a gene important in the immune system, a new study finds.
The particular variation occurs in nearly two-thirds of whites and blacks and about half of Asians, according to the study, which appears in the Oct. 9 issue of Nature.
The link between hepatitis and allergies is a dirty story -- literally. In one popular theory, called the "hygiene hypothesis," greater cleanliness makes people susceptible to allergies by reducing their exposure to germs, such as hepatitis A.
The liver virus thrives in unhygienic environments and large families. Before the 1970s, essentially everyone in the United States and other advanced nations had blood markers of exposure to hepatitis A. But as the Western world cleaned up its act and family sizes in industrialized countries shrank, the microbe got squeezed out. Now, only 25 percent to 30 percent of people in the developed world have signs of exposure to the virus.
But while hepatitis A was an unpleasant infection -- causing few or no symptoms in young children but jaundice, nausea, diarrhea and other complaints in adults -- contact with the virus seems to help the immune system mature. One piece of circumstantial evidence is that hepatitis A rates have fallen in the West while allergy rates have soared, doubling in the last two decades alone.
In previous work, a team led by Dr. Dale Umetsu of Stanford University found an immune system gene, called TIM-1, in mice. The gene is identical to a stretch of genetic material on chromosome 5 in humans known to play a role in susceptibility to allergies. They then found that some people have a slightly longer version of the area on chromosome 5 where the human version of TIM-1 may reside.
To see what effect, if any, the extra DNA has on allergies, Umetsu's group looked at the allergy histories of 375 men and women. People who'd been exposed to hepatitis A did indeed have fewer allergy problems -- but only if they had the longer version of the chromosome 5 region associated with allergies.
The researchers say it's not clear if a person must contract hepatitis A in childhood to gain protection against allergies, or if the contact can occur later in life. Another question is whether hepatitis A vaccines, which are available, match the effects of the natural virus on preventing allergies.
Dr. Paolo Matricardi, head of asthma and allergy research at the Children's Hospital in Rome, has been studying the connection between hepatitis A infection and allergies for several years.
"In our epidemiological studies we could not prove that there was a causal relationship between hepatitis A infection and [allergies]," Matricardi says. The latest work "opens up one very interesting potential biological explanation of our epidemiological observations."
However, Matricardi believes the apparent protection of hepatitis A infection may reflect the effect of exposure to other, yet unidentified organisms that travel in the same circles. "The more we investigate, the more we know that we know very little about the extremely complex relationship between infections and the development of allergic diseases," he says.