In order to further characterize human host defenses against Coccidioides immitis, extracts of this fungus were tested in vitro for their ability to attract polymorphonuclear leukocytes derived from peripheral blood of uninfected subjects. Soluble substances prepared from the mycelial (saprophytic) and spherule (tissue) phases exhibited, in the presence of serum, dose-dependent chemotactic activity. The dose-response correlations were different. The spherule-derived preparation showed decreased activity at the high concentrations, a diminution not observed with equivalent concentrations of the mycelial filtrate. Chemotactic activity was not observed with either substance in the absence of serum or if heat-inactivated serum was substituted. Because the response of human polymorphonuclear leukocytes to these fungal substances appears complement mediated, a selective cellular defect in this function which antedates exposure to C. immitis seems unlikely.

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