Several virulence factors aid Brucella in their survival inside macrophages and
other cells (84). The first and probably most important factor is the presence of the O-side chain, a linear polymer of mannose residues, on the lipopolysaccharide (LPS) of
smooth strains. The O-side chain is the most exposed antigen structure in Brucella
(22,26). The O-side chain of Brucella LPS induces a antibody-mediated response that is
somewhat protective in mice but not in cattle (26). In fact, it appears that production of
antibodies of certain subisotypes against Brucella O-side chain interferes with
complement activation. Interference in this process could then allow Brucella to survive
longer in cattle and set up a persistent infection (103).