Purified peritoneal exudate cells from rats infected once or twice with N. brasiliensis have been examined for the ability to produce the migration inhibitory factor (MIF) in vitro when cultured with living third-stage larvae. It was found that, 1 week after a single infection with 2,500 larvae, MIF production was readily demonstrable. The amount of detectable MIF in the culture supernatants tended to increase during the 2nd week but declined to below significant levels 3 weeks after infection. From the 4th to 8th week of infection, MIF production was highly variable. This was also the case for animals infected twice with 2,500 larvae where the interval between infections varied between 1 and 8 weeks, with 2 weeks between the second infection and harvesting of cells for culture. Cells able to produce MIF in culture could be inhibited to some extent by pre-treatment with serum from rats infected for 3 weeks, suggesting the presence of a serum inhibitor.

Rats infected 1–6 weeks with N. brasiliensis were tested at weekly intervals for delayed skin hypersensitivity and spleen cell migration inhibition. The antigen preparations used were extracts of larvae and worm tissue (somatic antigens) and metabolic antigens released into culture medium by living worms and larvae. Delayed skin reactions to and migration inhibition by the two adult antigen preparations were found from the 2nd or 3rd week of infection until the end of the experiment. No response could be demonstrated to the larval somatic antigens. With the larval metabolic antigens, a delayed skin reaction could be elicited during the 2nd and 3rd week of infection only, and the inhibition of spleen cell migration was significant from the 2nd to 4th week.