“…As early as 1894, it was known that the causative agent of plague, Yersiniapestis, is capable of forming a large capsule [1]. A major component of the capsule is the F1 (fraction 1) antigen -a polypeptide of 15.6 kDa, encoded by the fl operon [2,3]. The F1 antigen is highly immunogenic and has attracted most attention as a diagnostic tool [4] and component of protective vaccine [5].…”