The response of mice immune to Schistosoma mansoni to a challenge infection which bypasses the skin: evidence for two mechanisms of immunity.

Mice which had developed immunity to reinfection with Schistosoma mansoni following exposure to 20 cercariae and mice which had been immunized against S. mansoni by exposure to 400 highly irradiated (20 krad) cercariae, were tested for their ability to resist a percutaneous cercarial challenge and an intravenous challenge with 5-day-old lung-stage schistosomula derived from the same cercariae. Although both types of immune mice showed a marked resistance to a cercarial challenge, only the infected mice showed a comparable immunity to an intravenous challenge with lung schistosomula. These results confirm earlier studies which suggest that the major attrition of a cercarial challenge in infected mice occurs at the post-lung stage, whilst the attrition of a challenge infection in mice immunized with highly irradiated cercariae takes place in the skin. They provide further evidence for two separate mechanisms of immunity against S. mansoni in mice.