Viruses, virus-like particles are used to probe the protective capacity of the induced immune responses. Key problems that are addressed are how optimal T and B cell responses are induced and maintained and how they are influenced by Toll-like and other innate receptor stimulation. Induction and maintenance of memory B cells is a further subject of interest. Understanding the cellular networks of the immune system is of significant medical interest since i) prophylactic vaccines, which are the most effective medical intervention known to date, are based on the induction of long-lived immune responses and ii) a deregulated immune system is the underlying cause of many chronic diseases and autoimmunity.