An in vitro immune effector mechanism against the target excysted metacercariae of Paragonimus westermani was demonstrated in the rat system. Peritoneal exudate cells, mainly macrophages from normal rats, showed adherence to and killing of excysted metacercariae of P. westermani in the presence of complement-independent serum from rats infected with Paragonimus metacercariae. These reactions were specific for the excysted metacercariae, as tissue-migrating juvenile worms were not affected. Damage of excysted metacercariae of P. westermani due to antibody and macrophages was assessed by morphological observation, by cell adherence reaction and by the use of vital dyes. Trypan blue dye exclusion proved to be a reliable indicator of judging metacercarial viability. Electron microscopic studies demonstrated that macrophages reacted with fuzzy material on the tegumental surface and fine structures in the syncytium of the parasites. The tubular tunnels formed between the basement membrane and muscle layers of the damaged parasites were also noticeable. The relevance of these findings to cellular immunity in the early paragonimiasis was discussed.
