Keratin at the Dental–Systemic Interface: A Bidirectional Translational Analysis and Narrative Review

General medicine has accumulated over a decade of mechanistic, processing, and immunological knowledge about keratin, almost entirely grounded in skin and long-bone physiology. Dentistry is now independently “rediscovering” keratin, largely ignoring this existing evidence base while simultaneously confronting oral-specific challenges that general medicine has never encountered. The value of this review lies in systematically connecting the two evidence chains for the first time and arguing that the oral cavity is not merely another application site but a unique biological opportunity capable of conceptually feeding back into general medicine’s understanding of keratin. Keratin is not a general medicine material awaiting dental application. It is a native enamel component whose developmental biology is still being characterized, and the oral cavity is a biological stress test of unusual severity.