As the swine flu is turning into a global pandemic, important clues about why influenza is more severe in some people than it is in others, have been discovered by scientists.
Researchers at the Children's Hospital of Philadelphia, Pennsylvania, have shown that the influenza virus can actually paralyse the immune systems of otherwise healthy individuals, which could lead to severe secondary bacterial infections, such as pneumonia.
Also, this immunological paralysis could be long-lived, which is important to know while developing treatment strategies to combat the virus.
"We have a very limited understanding of why some people who get influenza simply have a bad cold and other people become very sick and even die. The results of this study give us a much better sense of the mechanisms underlying bacterial infections arising on top of the viral infection," said Dr. Kathleen Sullivan.
For the study, the researchers enrolled paediatric patients with severe influenza, and examined the level of cytokines, which serve as the first line initiators of immune response, in the blood plasma.
Although the researchers found elevated levels of cytokines, they also found a decreased response of toll-like receptors, which activate immune cell responses as a result of invading microbes.
Their findings suggested that the diminished response of those receptors could lead to the paralysis of the immune system, and ultimately cause secondary bacterial infections.
The influenza patients were compared with patients with moderate influenza, respiratory syncytial virus, and a control group of healthy individuals.
It was found that the immune paralysis could have specifically resulted from influenza infection and was not seen in patients with respiratory syncytial virus.
The researchers say that their work might explain why one quarter of children who die from influenza, die from a bacterial infection occurring on top of the virus.
The study has been published online in the Journal of Leukocyte Biology.