1. An interactive information retrieval system, including non-transitory computer-accessible storage media, that searches a database of content items, the system comprising: a. a multi-prefix search module, embodied in a first non-transitory computer-accessible storage medium, that receives keystrokes as they are entered by a user into a mobile device, searches for content items within the database that contain words having a prefix matching the keystrokes, and generates a first-tier set of search results containing matching content items; b. a first-tier search result delivery module, embodied in the first non-transitory computer-accessible storage medium, that delivers the first-tier set of search results to the mobile device for display to the user; c. a client application, embodied in a second non-transitory computer-accessible storage medium, running on the mobile device that generates, based on the content of at least one of the first-tier set of search results, at least one dynamic menu item that includes at least one second-tier search field, wherein additional keystrokes entered by the user into the second-tier search field are delivered to the prefix search module, wherein the prefix search module searches at least one channel of content items within the database that contain words having a prefix matching the additional keystrokes, and generates a set of second-tier search results containing matching content items; and d. a second-tier search result delivery module, embodied in the first non-transitory computer-accessible storage medium, that delivers the second-tier set of search results to the mobile device for display to the user.