Reaction of Human Immunoglobulin G against K. pneumoniae in the Presence of Complement

The bacteriolytic action of human immunoglobulin G (IgG) and its modifications (pepsin-treated and sulfonated IgG) in the presence of complement was examined with K. pneumoniae and S. aureus. Lysis of K. pneumoniae was observed when concentrated IgG was added, while that of S. arueus was not. This lytic action against K. pneumoniae was unaffected by the above modifications of IgG. The results raise the possibility that IgG antibody an high concentration causes the lysis of K. penumoniae by activating the alternative pathway of the complement system and that such activation requires antibody specific to K. pneumonia.