SUMMARY. The bactericidal action of serum from 61 adult patients with cystic fibrosis (CF) against autologous and heterologous strains of has been studied. CF serum had a similar bactericidal action to normal human serum (NHS) against a reference panel of strains. Six CF sera had a selective inability to kill autologous strains of pseudomonas, which were sensitive to NHS and to sera from other CF patients. The six sera had normal levels of complement and immunoglobulin and were bactericidal to other strains. A titratable blocking factor was present in these sera and it interfered with the bactericidal action of NHS on the appropriate strain. This factor was present in the IgG-containing fractions of serum obtained by ion-exchange chromatography, but was not removed from the serum by absorption with the pseudomonas strain. Some CF sera may fail to kill sensitive strains of pseudomonas because of the development of a blocking IgG antibody against naturally occurring bactericidal IgM antibody.


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