Primary infection sera and IgG1 do not block host-protective immunity to Nematospiroides dubius

IgG1 immunoglobulin purified from sera 3 weeks following primary infection with 400 L3 larvae of Nematospiroides dubius was shown to be protective against challenge infection as assessed by worm recovery and worm length. No evidence was seen of blocking activity either using primary infection sera or IgG1 purified from primary infection sera in a variety of assay systems both in vivo and in vitro. In addition, immune complexes precipitated from primary infection sera by polyethylene glycol failed to inhibit the capacity of immune mesenteric lymph node cells to transfer immunity. It is concluded that an explanation for the prolonged survival of Nematospiroides dubius must be sought in mechanisms other than circulating blocking factors.