Die sellulere verdedigingsmeganisme by die varswaterslak Bulinus Africanus

Abstract

Certain aspects of the cellular immune mechanism of the freshwater snail Bulinus africanus were investigated. It was established by means of electronmicroscopy that mainly two cell types namely granulocytes and hya linocytes circulate in the haemolymph. The in vitro morphological changes of the granulocytes were not dependent on the presence of snail serum. Although there was a correlation between snail mass and cell number no such correlation could be found between the size of either the granulocytes, or the hyal inocytes and snail mass. Fluorescence microscopy was used to investigate the possibility of granulocyte subpopulations in the haemolymph of this snail. This was done by using lectins isolated from Glycine max, Canavalia ansiformis, Ricinus communis en Triticum vulgaris. No subpopulations could be found. The functional properties of the g ranulocytes and hyalinocytes were investigated by both light and electron microscopy. Only the granulocytes were involved in the phagocytosis of Serratia marcescens and Staphyllococcus aureus bacteria and Saccharomycas cerevisiaa yeast cells. In the encapsulation of Schistosoma haematobium and Schistosoma mansoni sporocysts only granulocytes were involved. Although the influence of humoral factors in these functions could not be excluded, it seems that hemagglutinin as a humoral factor was not involved. In an investigation of the influence of oxygen-free water, snail excretion products , and immobilization conditions on leucocytosis only oxygen-free water led to such a reaction. The granulocyte response upon trematode infection showed a decrease in the cell number two hours after infection, which in the case of a first

infection returned to normal after 42 days. In the case of a second infection with a non-compatible parasite this recovery was evident after only 14 days showing the possible existence of a memory mechanism. Two techniques, namely 51 Cr-radiolabelling and histological processing were performed in order to confirm the penetration of trematode larval stages into the snail. Although it was established that 51 Cr could not be used in the radio-labelling of the parasites, the parasites were observed in the snail's headfoot tissue by means of histological processing.