Importance of antiserum and phagocytic cells in the protection of mice against infection by Klebsiella pneumoniae.

Rabbit antiserum to Klebsiella pneumoniae showed a powerful protective effect against intramuscular infection in normal mice. No protective effect was observed in mice whose monocytes and polymorphonuclear cells were depleted by X-irradiation. The antiserum had approximately the same protective effect in mice whose macrophages were blocked selectively by carrageenan as in normal mice. It is concluded that antiserum exerted its effect by opsonic function and that opsonized K. pneumoniae were eliminated mainly by polymorphonuclear cells rather than macrophages, at least in an early phase of the infection. these findings were supported by histological examination and observation of intracellular killing in vitro.