A study suggests that people are made critically ill, or even killed, by their own immune response during H1N1 flu pandemic.

Fernando Polack, associate professor of paediatrics at the Vanderbilt University Medical Centre, and colleagues in Argentina and Nashville provide a possible explanation for this alarming phenomenon of pandemic flu, according to a Vanderbilt statement.

"Every time there is an influenza pandemic, there is a large proportion of younger, or middle-aged adults who die. We have always explained these deaths, based on presumed virulence of virus, or getting bacterial infection at the same time," Polack said.

During the flu pandemic, Polack and his team went to work looking for evidence of a biomarker he had used before.

A biomarker is a protein that can be measured in blood or tissue whose concentration reflects the severity or presence of some disease state.

Polack found high levels of C4d in lung tissues that suggest a massive, potentially fatal activation of the immune system.

Todd Rice, assistant professor of Medicine at Vanderbilt, has seen people getting killed by the "exuberant" and uncontrolled response of the immune system in other diseases - like sepsis.