It has been previously reported that a filterable microorganism belonging to the genus Leptospira has been recovered from the blood or organs of human beings suffering from the disease known as yellow fever in Guayaquil, and that the organism, which has been termed Leptospira icteroides , induces in certain experimental animals the characteristic symptoms and lesions observed in the patients from whom it was isolated. It has also been previously shown that the serum from patients recovering from an attack of yellow fever in Guayaquil had the power to agglutinate and dissolve the organism when introduced into the peritoneal cavity of a normal guinea pig (Pfeiffer phenomenon). Moreover, the guinea pigs which had once been inoculated with the blood of yellow fever patients without succumbing to the infection, notwithstanding the fact that they had shown a definite febrile reaction after 4 to 5 days, were found to be refractory to a subsequent inoculation of a culture of Leptospira icteroides All these observations pointed to the possible relation of this organism to the disease known as yellow fever in Guayaquil. The demonstration of the filterability of the organism and the transmission of the infection with the same organism by Stegomyia calopus have further strengthened the probable etiological significance of the organism in yellow fever.It was by no means a simple problem to determine the relation existing between Leptospira icteroides and Leptospira icteroh?morrhagi? . An experiment reported in a previous paper seemed to justify the view that the two leptospiras are closely related but not identical, yet it was necessary to exhaust various other modes of differentiation before the distinction between them was firmly established. The present paper continues this phase of the inquiry in further detail.There have been taken up here the phenomena of agglutination, the reaction of Pfeiffer, complement fixation, the protective properties of various monovalent and polyvalent immune sera, and active immunity. As the result of experiments in connection with these immunity phenomena the following data are presented.Monovalent immune sera prepared by several successive injections in an animal naturally refractory to Leptospira icteroides possess the power to agglutinate in vitro not only the homologous strains, but also all other strains of icteroides tested. On the other hand, a slight effect, or none at all, has been observed when these immune sera have been mixed in vitro with various strains of Leptospira icteroh?morrhagi? . A similar relation exists between the monovalent anti- icteroh?morrhagi? sera and the various strains of Leptospira icteroides; that is, there is a slight agglutinating effect in some instances upon the icteroides strains, but it is never so strong as that occurring in tests against the icteroh?hagi? strains. The Pfeiffer reaction gave a sharper differentiation between the two groups, for in most instances the phenomenon was specific for the group. There were occasional doubtful reactions, but not enough to warrant a confusion of the two groups.Polyvalent immune sera, one specific for icteroides , and the other for icteroh?morrhagi? , showed a high titer of neutralizing power for the cultures of the homologous groups. It was found, however, that the action of the sera is by no means absolutely specific, because the injection of a sufficient amount of the anti- icteroides serum apparently prevented a fatal outcome in a guinea pig inoculated with multiple minimum lethal doses of a culture of Leptospira icteroh?morrhagi? , and vice versa . The specificity of the serum was demonstrated only when it was used in smaller quantities.More or less specificity was shown by the complement fixation reaction, but it was not absolute. Weak fixation occurred when the anti- icteroides serum was mixed with one or the other of the icteroh?morrhagi? strains and vice versa , and strong fixation occurred only when the antiserum was
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