A Study of the Resistance of Normal Human Beings to Recently Isolated Strains of Pathogenic Pneumococci

With a view to obtaining information as to the virulence of pneumococci for human beings a study was made of the pneumococcidal action of normal human serum-leucocyte mixtures for freshly isolated strains of pathogenic pneumococci. It was found that human beings as a group showed well marked pneumococcus destroying power in their blood for all types of organisms studied. Individuals, however, exhibited wide variations in their reactions against the different types. These ranged from marked killing effect for one type of pneumococcus to none or slight against another. While reactions against different strains within the type often varied considerably this difference was less, on the whole, than that between types. An interpretation of these findings in the light of previous animal experiments in which actual determination of resistance to pneumococcus infection was made leads to the inference that human beings in general possess a considerable degree of natural immunity to all types of pneumococci but that individuals may be relatively susceptible to one or more types and at the same time resistant to others; also that pathogenic strains of pneumococci vary much in their virulence for man.