Y. pestis has a number of virulence factors that enable it to survive in humans by facilitating use of host nutrients, causing damage to host cells, and subverting phagocytosis and other host defense mechanisms. Capsular F1 antigen is a main immunochemical component of Y. pestis surface. Its synthesis is determined by pFra plasmid and induced at 370C. F1 is aggregated on the outer membrane of the Y. pestis microbe as an olygomeric protein, forming a granular layer and gradually diffusing into the environment. Capsular polymer consists of multiple similar hydrophobic protein subunits, aggregating at physiological conditions. F1 subunit of the antigen is originally synthesized from 170 amino acids and has a molecular mass 17.6kD, then, after the elimination of the signal peptide during the secretion onto the surface, a protein with molecular mass 15.6kD and isoelectric point 4.1 is formed.