A study led by Dr Ben Nicholas, of the University of Southampton, and published in the Journal of Allergy and Clinical Immunology, assessed lung samples from asthmatics and healthy volunteers.
The samples were exposed to influenza and their reactions analysed.
Dr Nicholas, who led the study under the NIHR Southampton Respiratory Biomedical Research Unit, said: "We wanted to look into whether immune system differences explain why asthmatics are more likely to end up in hospital if they get flu than the general population. This is important, as flu can cause a person's asthma symptoms to get worse. The samples from healthy people showed a strong immune system-triggering reaction to the flu virus. But in lung samples from asthma patients, this reaction was much weaker.
"We hope these results help researchers better understand why asthmatics are more affected by influenza and help find new treatments for common lung infections, which often make asthma symptoms worse."