Light and electron microscopic appearance of rat peritoneal mast cells adhering to schistosomula of Schistosoma mansoni by means of complement or antibody.

Metrizamide-gradient purified rat peritoneal mast cells were allowed to adhere to schistosomula of S. mansoni that had been preincubated in either minimum essential medium with 5% fetal calf serum (MEM/FCS), heat-inactivated serum from rats 8 weeks after cercarial infection (IRS) fresh normal rat serum as a source of complement (NRS), or IRS and NRS. Adherence was evaluated in sections 0.3 micrometers thick. The preincubation conditions favoring cell adherence were IRS + NRS greater than NRS greater than IRS greater than MEM/FCS. Greater adherence was seen at 60 min than at 10 min. Discharge of mast cell granulates was seen infrequently, was not greater in adherent than nonadherent cells, was not increased by any preincubation condition, and did not occur against the parasite's surface, as would be expected if antigen, antibody, and surface receptors were aggregated there. Electron microscopic examination showed that attachment to the parasite occurred through electron-dense material which was usually fibrillar in appearance. Membrane fusion, such as is seen between human neutrophils and schistosomula, did not occur.