Y.pestishasanumberofvirulencefactorsthatenableittosurviveinhumansbyfacilitatinguseofhostnutrients,causingdamagetohostcells,andsubvertingphagocytosisandotherhostdefensemechanisms.CapsularF1antigenisamainimmunochemicalcomponentofY.pestissurface.ItssynthesisisdeterminedbypFraplasmidandinducedat370C.F1isaggregatedontheoutermembraneoftheY.pestismicrobeasanolygomericprotein,formingagranularlayerandgraduallydiffusingintotheenvironment.Capsularpolymerconsistsofmultiplesimilarhydrophobicproteinsubunits,aggregatingatphysiologicalconditions.F1subunitoftheantigenisoriginallysynthesizedfrom170aminoacidsandhasamolecularmass17.6Kd,then,aftertheeliminationofthesignalpeptideduringthesecretionontothesurface,aproteinwithmolecularmass15.6Kdandisoelectricpoint4.1isformed.