The role of serum in leucocyte adherence to the plerocercoid ofLigula intestinalis(Cestoda: Pseudophyllidea)

SUMMARY The role of serum in the adherence of roach (Rutilus rutilus) leucocytes to the plerocercoid ofLigula intestinalishas been investigatedin vitro. Roach plerocercoids, either untreated, cultured or killed (fixation in 0·5 % buffered glutaraldehyde or heat at 50°C for 45 mm), were exposed to serum and 4×106leucocytes obtained from the pronephros of non-infected roach using a discontinuous Percoll gradient. At normal roach serum (NRoS) dilutions of 1:5000 to 1:100, leucocyte adherence was observed on all living parasites but was negligible in killed parasites or in complement-depleted NRoS assays. The number of adherent cells increased with increasing NRoS concentration. Leucocytes bound more avidly in the presence of heat-inactivated immune serum (hi IRoS); at a serum dilution of 1:5000 the percentage of the parasite surface covered by leucocytes was approximately 3 times greater in hi IRoS than in NRoS. Ultrastructural observations on parasites revealed that plerocercoids exposed to cells and hi IRoS had, in some areas, abnormal inclusions in their tegument and lacked a microthrix border and distal cytoplasm. In addition, the efflux of radioactivity from [14C] cycloleucine-labelled worms was greater in parasites incubated in the presence of hi IRos.