New insights is being provided by a study on human saliva as why older people were better able to fight off the new strains of "bird" flu and "swine" flu than younger people.
Zheng Li and colleagues explain that saliva does more than start the process of digesting certain foods.
Saliva also contains germ-fighting proteins that are a first-line defense against infections.
Scientists already knew that levels of certain glycoproteins - proteins with a sugar coating that combat disease-causing microbes - differ with age.
Li's team took a closer look at how those differences affected vulnerability to influenza.
Their tests of 180 saliva samples from men and women of various ages suggested that seniors, who fought off the bird flu better than the younger groups, might thank their saliva.
Glycoproteins in saliva of people age 65 and over were more efficient in binding to influenza than those in children and young adults.
The research "may provide useful information to help understand some age-related diseases and physiological phenomenon specific to women or men, and inspire new ideas for prevention and diagnosis of the diseases by considering the individual conditions based primarily on the salivary analysis," the scientists said.
The study is published in the ACS' Journal of Proteome Research.