An American study has said that people with allergic asthma face double trouble during flu season.
The research, conducted by University of Texas Southwestern Medical Center, appears in the June edition of the Journal of Immunology.
Lead author Dr. Michelle Gill, assistant professor of paediatrics and internal medicine at UT Southwestern, said: "Our findings imply that the better your asthma is controlled, the more likely you are to have an appropriate response to a virus.
Fifty-six people ranging in age from 3 to 35 participated in the study.
Twenty-six of the participants suffered from allergic asthma; the remaining 30 made up the control group. Most of the participants were African-American, and the mean age was 15 years in both the asthma and control groups.
Moreover, those in the asthma group had been diagnosed by a physician and had a positive skin test to at least one indoor allergen.
Researchers first isolated immune cells called dendritic cells from study participants.
These cells are found in blood and tissues that are in contact with the environment, such as skin and the linings of the nose and lungs.
When they encounter respiratory viruses such as flu, dendritic cells normally produce proteins that help the body mobilize the immune system and overcome the viral infection.
When the dendritic cells first encounter an allergic stimulus, however, they are significantly impaired in their ability to produce such antiviral proteins.
When investigators exposed the dendritic cells from the study participants with allergic asthma to influenza, they found that the cells were unable to produce interferon, an immune system protein that plays a key role in fighting off repeated infections of the same virus.
Interferon is what makes a person feel run down and tired when fighting viral infections.
The researchers speculate that the immune-suppressing effect of the allergic stimulation of dendritic cells might be related to the high levels of a molecule called IgE normally found in people with allergic asthma.
Among the subjects participating in this study, elevated IgE levels were associated with impaired capacity of dendritic cells to produce interferon when exposed to flu.
Dr. Gill said: "These findings imply that allergic triggers associated with exposure to indoor allergens like pet dander and dust mites can potentially render cells deficient in responding to a virus.
"It may - although this remains to be proven - also explain why asthmatics who are sensitive to indoor allergens often experience asthma exacerbations when they acquire respiratory viral infections."