In mice, the presence or absence of a single complement (C') component, called hc1, is controlled by two alleles at the Hc locus. The sera of mice which lack this C' component do not manifest C'-mediated immune hemolysis. When challenged with the common mouse pathogen, Corynebacterium kutscheri, mice possessing hemolytic C' fare slightly better than C'-deficient mice. When mice harboring latent C. kutscheri are administered hydrocortisone, which depresses mouse serum C' levels, pseudotuberculosis is activated with equal frequency in mice of both C' types. These data suggest that in at least one situation the presence of the complete hemolytic C' system may be advantageous to the mouse. In contrast, evidence is presented which shows that under normal laboratory conditions, C'-deficient B10.D2 "old line" mice (Hc0/Hc0) have a survival advantage over C'-positive B10.D2 "new line" mice (Hc1/Hic1) during the first 3 wk of life. It is therefore concluded that mouse hemolytic C' has a balanced survival value—that is, under one set of conditions it may be advantageous, whereas in another situation, it may be disadvantageous.

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