Abstract
Evidence obtained by others who used direct immunofluorescence staining to demonstrate serological differences among strins of Legionnaires disease bacterium prompted this study of parameters influencing the ability of the indirect immunofluorescence test to detect human antibodies to Legionnaires disease bacterium. A total of 25 Legionnaires disease bacterium strains, representing 4 serogroups, were used as immunofluorescence antigens to test selected human sera. The use of diethyl ether in preparing the antigens was discontinued when it was found that titers against ether-killed group 2 (Togus 1-like) antigens were impossible to determine. Instead, heat-killed suspensions of Legionnaires disease bacterium in 0.5% buffered normal chicken yolk sac were used to show the serogroup diversity of the strains and the serogroup specificity of the antibody response of some, but not all, patients with serological evidence of Legionnaires disease. Multiple antigens should be used in serological tests for Legionnaires disease. The fact that some sera contain antibodies that bind equally well to strains of all 4 serogroups implies that demonstration of a 4 fold increase in titer of paired sera when tested with a single antigen should not be interpreted as evidence of infection with a strain of the same serogroup.

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