During flu season, a strong immune system is a person’s first line of defense against harmful bacteria and viruses. To boost immunity, enrich the diet with fruits and vegetables high in antioxidants to help fight free radicals; eat less fat and animal proteins; manage stress; and get enough sleep. Green tea and herbs such as astragalus, echinacea, ginseng and licorice root act as immune boosters, by both providing antioxidants and stimulating protective enzymes in the body.

 
Sources: My.ClevelandClinic.org, Healthy.net