Marek's disease (MD) is a lymphoproliferative disease of chickens caused by a herpesvirus (MDV). Several effective
vaccines have been developed and MD is therefore often considered as a model for studying antitumour
vaccines. Key factors for the understanding of
vaccine-induced immunity are discussed. Three serotypes have been characterized: serotype 1 or oncogenic MDV, serotype 2 or non-oncogenic MDV and serotype 3 or herpesvirus of turkeys. The three serotypes have clearly different genomes and
proteins. The pathogenesis of
infection with serotype 1 MDV can be divided into an early cytolytic phase, a latent phase and a second cytolytic
infection combined with the development of tumours and permanent immunosuppression. The activation of T cells during the early cytolytic phase is important for establishing
infection in the target cells for latency and transformation. Immune responses by B cells, and especially by T cells, are directed against
viral infection. Evidence is presented that
Marek's disease tumour-associated
surface antigen is not involved in the antitumour responses. Moreover, the importance of antitumour immune responses in MD is questioned. Vaccinal immunity is dependent more on T cells than B cells and is directed against virus
antigens. It is proposed that the evidence claimed for an antitumour response induced by vaccination actually relates to an
antiviral response. The relevance of new, highly oncogenic strains causing MD in vaccinated chickens is discussed.