Most strains of Klebsiella pneumoniae activate Complement and bind C3b but resist its bactericidal activity. Serum-resistant strains activate Complement through the alternative pathway, whereas serum-sensitive strains also activate the classical pathway. Outer membrane porins activate both Complement pathways, and the rough lipopolysaccharide activates the classical pathway, causing cell lysis. In serum-resistant strains, these structures are covered by the O-side chain of the lipopolysaccharide, thus preventing insertion of Complement components in the outer membrane. A minority of strains express capsular serotypes that cover porins and lipopolysaccharide. Since these capsules themselves do not activate Complement, strains are serum-resistant.