Misdirected immune response causes more severe illness second time around.

2003 JUL 9 - (NewsRx.com & NewsRx.net) -- While some diseases leave the patient with immunity against future attacks, people with dengue fever get sicker the second time around.

Scientists now believe they know why, and their finding may help lead to ways to fight the dangerous mosquito-borne illness.

The problem is "original antigenic sin" on the part of the immune system, according to a team led by Gavin Screaton of the MRC Human Immunology Unit at Weatherall Institute of Molecular Medicine at John Radcliffe Hospital in Oxford, England. Antigens battle diseases invading the body.

There are four subtypes of dengue, and people infected with the disease mount an immune response to fight off the version that infects them. If a patient later acquires a different subtype, however, the immune system mounts an attack directed at the original form of the disease, the researchers found in a study of volunteers in Thailand.

That means they produce immune cells that are less effective against the new form, which can then attack the victim more strongly, the team reports in a paper in the July 2003 issue of the journal Nature Medicine. This misdirected immune response barely fights off the new infection. What's more, some immune cells seem to self-destruct, exacerbating the problem.

Dengue fever causes severe headaches, fever, and rashes. The more serious dengue hemorrhagic fever has a fatality rate of 5%, according to the U.S. Centers for Disease Control and Prevention. The virus is estimated to affect as many as 50 million people annually in tropical areas and kill 12,000 or so. No vaccine or specific treatment has been found.

Having the body respond to the wrong form of the dengue virus complicates the process of developing a vaccine. That's because a vaccine uses a dead or weakened virus to mimic an infection, so the body will recognize the ...