Final answer:
The F1 capsular antigen is expressed exclusively by the bacterium Yersinia pestis, which causes plague.
Step-by-step explanation:
The F1 capsular antigen is a component of Yersinia pestis, the causative agent of plague. It is expressed exclusively by this bacterium when it is in the mammalian host and is known for its antiphagocytic properties, contributing to the virulence of the pathogen. On the other hand, the type of antigen-presenting molecule found only on macrophages, dendritic cells, and B cells is Major Histocompatibility Complex (MHC) class II. B-cell receptors (BCRs), which are membrane-bound IgD and IgM antibodies, bind specific antigen epitopes with their Fab antigen-binding regions but are distinct from the F1 capsular antigen.