B- and T-cells play crucial roles in eradicating viral infections. The focus of this project is two-fold. First, to study general in vivo mechanisms of virus - host interactions and second, to understand how primary and memory B- and T-cells respond to virus infection. Despite the importance of their effector functions, we do not yet fully understand the induction of B and T-cell immunity. Specifically, for B-cells we do not know the mechanisms responsible for B-cell immunodominance. For T-cells, especially for viruses in which traditional vaccine approaches have not been successful, it is hoped that induction of T-cell memory may enhance protective immunity. To induce optimal T-cell responses to vaccines it is necessary to understand how primary T-cells are stimulated. This project aims to address questions related to the optimal activation of B- and T-cell immunity following primary and secondary virus infection by studying several viruses and different routes of infection. In 2015, we have developed a simple flow cytometry based method for measuring B-cell surface immunoglobulin avidity. This assay enables measurement of the affinity maturation of the B cell response to influenza A virus.