Physiological characterization of a biphasic immune response to Trichinella spiralis in the rat.

Secondary infection with Trichinella spiralis in the rat was associated with a host response that prevented approximately 80% of the total number of larvae recovered after primary infection from embedding in the intestine. The host response occurred within 15 min after contract of the parasite with intestinal mucosa and was not associated with acute inflammatory cell infiltrates or with the pathophysiology of the primary infection. Eventual rejection of the residual population of embedded worms occurred between four and eight days after infection and involved tissue changes similar to those attending worm rejection during primary infection. Elimination of enteric stages of T. spiralis in primary infection probably involves a single response aimed at the expulsion of established worms. In contrast, immune rejection of worms in secondary infection involves physiologically and presumably immunologically distinct "early" and "late" responses, with each response having a different developmental stage of the parasite as its target.