The present invention provides an immunogen composition and methods for using the same for the development of immunity, and particularly at mucosal sites in a mammal, thereby providing immunity at the site of entry for many major pathogenic organisms and also systemic immunity. The immunogen composition includes an antigen, a biocompatible polymer, and a liquid vehicle, with the biocompatible polymer and liquid vehicle being present in such proportions and interacting in such a way that the immunogen composition exhibits reverse-thermal viscosity behaviour. A delivery vehicle composition including a drug other than an antigen is also provided. Methods are provided for delivering the compositions of the invention to a host.