The term antigen [bacterin] is a compound word. It is formed from "anti" which means against and gene which means be able to produce the anti. Compete in its general sense indicates a try to win as you concore the others in a race or an interview. In combined bacterin sense, the term bacterin competition means a bacterin try to compete the other(s) in combination of bacterins for interaction with macromolecular structures, cell or for interplay in cell - cell events of immune responses. Such competition take part in several immune and bacterin functions. Since the date of emergence of immunology as a science, several immune phenomena are being sounded outstanding. These phenomena are spanning as; zonation, cross-reactivity, blocking antibodies, immune cross-roods and antigenic competition. Antigenic competition is involved in immunology and vaccinology. So far vaccinology is concerned, antigenic competition interplayed with immunity to infection, immune regulation, immunity of vaccine, vaccine preparation and development and combined vaccine and vaccinations. The book is intended to serve immunology and vaccinology students as well as researcher in this field.