Following the oral administration of third- and fourth-stage A. suum larvae, which had previously completed nine or ten days somatic migration in guineapigs, rabbits or a pig, six out of eight pigs showed a rise of serum antibody titre, as measured by the conglutinating complement absorption test using a saline extract of Ascaris worm as antigen. Four of these pigs had negative titres before infection, and in these animals antibodies were first detected between the 14th and 20th days. Maximum serum antibody concentration was reached between the 14th and 34th days of infection, and antibodies were detected for at least as long as 68 days.

Larvae were recovered from the faeces on the 23rd and 29th days of infection, and in two pigs out of eight the larvae reached maturity, eggs first being seen in the faeces on the 42nd day.

The results suggest that (1) Ascaris larvae, on their return to the intestine, release antigenic substances which are then absorbed by the gut wall to stimulate the production of antibodies; (2) the secondary antibody rise, which occurs about the sixth or seventh week of an infection with Ascaris eggs, is due to the absorption of this larval antigen; and (3) the moulting process (fourth moult) is not only an important stage for stimulating this further antibody production, but is also the time when maturing larvae are affected by the immune mechanisms of the host.

On reinfection with Ascaris eggs the phenomenon of “self-cure” was noticed. A fall in the faecal egg count, which became negative on the 21st day, was accompanied by a sharp rise in antibody titre. An increase in the number of infertile eggs in the faeces, rising from 11% on the day of reinfection to 100% on the 14th day, was also observed.