Influenza A is characterized by seasonal outbreaks and a gradual genetic, yet discontinuous antigenic evolution (antigenic "cluster jumps" occuring every 4-7 years, with a large seasonal epidemic as a result of lacking immunity among the population).The interplay between viral mutations and an age-group-specific human immune response is modelled here, trying to account for the observed phenomena and to predict next season's epidemic strain from the dynamics of the previous year.