It has been considered that the polysaccharide capsular material from microorganisms such as Klebsiella pneumoniae induces mainly thymus-independent humoral immunity, nevertheless studies done with Bacteroides fragilis have shown the participation of cellular immune effector mechanisms. Works done to define the immunogenic role of the capsular polysaccharide of Klebsiella pneumoniae indicate that both active and passive immunization with these antigens confer protection against infections with this bacteria. Several studies propose the use of vaccines prepared with one or various capsular serotypes of Klebsiella pneumoniae in immunoprophylaxis or immunotherapy.