Schistosomiasis from S. mansoni in mice: the relationship between acquired immunity and serum levels of lethal antibody.

Acquired immunity to Schistosoma mansoni induced by a primary infection with cercariae of one or both sexes was assessed in mice by the recovery of parasites of a challenge infection from the lungs or the liver. In addition, the antibody-dependent complement-mediated cytotoxicity against schistosomula in vitro was titrated in sera obtained from both groups of animals. The degree of immunity as detected by the lung recovery technique in mice infected with a mixture of male and female cercariae was variable and lower than 50%. In contrast, no immunity was observed in the group of animals infected with 40 'single-sex' cerariae. The titre of lethal antibody in a pool of sera firm these animals was about 10, whereas it was about 640 in a pool of sera from bisexually infected animals. Lethal antibody titres ranged from 20 to 120 for unisexual infection and 240 to 1920 for bisexually infected mice. In some cases a significant degree of immunity was detected by liver perfusion in mice with a primary unisexual infection. However, the lethal antibody titre of sera from mice with a 'single-sex' infection remained low, even when the immunity was apparent.