F1 antigen

Yersinia pestis is a gram-negative coccobacillus belonging to the Enterobacteriaceae. Yersinia pestis is primarily a rodent pathogen, with humans being an accidental host when bitten by an infected rat flea. The flea draws viable Y. pestis organisms into its intestinal tract. Some Y. pestis in the flea are then regurgitated when the flea gets its blood meal thus transferring the infection to a new host. While growing in the flea, Y. pestis loses its capsular layer, which is made of the F1 (or fraction 1) capsule antigen. Most of the organisms are phagocytosed and killed by the polymorphonuclear leukocytes in the human host. A few bacilli are taken up by tissue macrophages. The macrophages are unable to kill Y. pestis and provide a protected environment for the organisms to synthesize their virulence factors.