Abstract
Previous work by Robertson and Sia 1 has shown that the blood (serum and leucocytes) of certain pneumococcus-resistant animals possessed destructive properties for pneumococci not found in the blood of susceptible animals. This suggests that the natural immunity of pneumococcus infection depends chiefly, if not entirely, on the pneumococcidal activity of the blood. In their studies pneumococci of high virulence only were employed, and the animals tested represented well marked examples of natural immunity and susceptibility, the dog and cat on one hand, and the rabbit and guinea pig on the other. It is a commonly observed fact that within a single species of any of the usual laboratory animals there occur wide variations in susceptibility toward different strains of disease-producing pneumococci, and that among the so-called susceptible animals such variations may be extreme.
Experiments were undertaken with the purpose of determining whether the immunity shown by a relatively susceptible animal, as the rabbit, against certain strains of pneumococci was associated with demonstrable pneumococcus-destroying powers in the blood. The technique employed was the same as that used by the above mentioned workers. This consists, briefly, in seeding varying quantities of pneumococci into mixtures of rabbit serum and leucocytes contained in small tubes, which are then sealed with paraffined corks and attached to an apparatus inside the incubator whereby constant agitation is carried on during incubation.
It was found that the rabbit serum-leucocyte mixtures possessed the power to kill avirulent pneumococci in relatively large numbers but failed to inhibit the growth of virulent organisms even in minute quantities. The results of numerous experiments in which all three types of pneumococci were employed indicated that the ability of a strain of penumococcus to grow in rabbit's blood is dependent on its virulence for the rabbit.
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