An established adult infection of Nematospiroides dubius was unaffected by the administration of immune lymphocytes and immune sera whereas an incoming larval infection was expelled. Past experiments have shown that the immune inoculum at least had the ability to recognize adult stages, leading to the hypothesis that adult stages secrete or excrete an immunomodulatory shield around themselves in the gastrointestinal tract. This hypothesis was given further credence by the demonstration that soluble antigens derived from adults abolished the generation of homologous immunity to this parasite. Modulation of immunity was reflected by increased fecundity, increased worm size, and increased survival time in the gut.