Protective immunity to Nippostrongylus brasiliensis in the rat. II. Adaptation by worms.

Adapted worms are the adult worms found in immune rats reinfected with Nippostrongylus brasiliensis. They are adapted because, when transferred from the small intestines of donor rats to the intestines of normal rats, they persist for a much longer period than normal worms transferred from initially infected donors. When transferred, the internal structure of adapted worms is disorganized and they are permanently stunted but these factors do not account for their longevity in this experimental situation. The adaptation occurs during the larval stages of parasitic life beginning at the L3/L4 moult. Rats were passively immunized with numerous pools of antisera and infected with normal or adapted worms. Normal worms were expelled from the rats by all the pools of sera tested but only one pool expelled normal and adapted worms with equal effectiveness. In most cases the adapted worms were only slightly or not at all affected by passive immunity, although adapted worms did not regenerate their internal structure in passively immunized rats. The different response of adapted and normal worms to the host's immunity probably results from a change in the antigenicity of adapted worms.