The internal defense system consists of soluble components of hemolymph and circulating cells known as hemocytes. The circulating hemocytes play a central role in innate immunity. This work aimed to study the hemocytes of both susceptible and resistant B. alexandrina snails exposed to S. mansoni infection using light and electron microscopes. Two tested groups were included in the study; 60 susceptible and 60 resistant B. alexandrina snails. Both tested groups were studied as regad the hemocyte count (before and after infection) and the morphological characteristics of both circulating and tissue hemocytes by light and electron microscopes. Before infection, there was no significant difference between the two groups as regard the hemocyte count, however after infection, there is a significant decrease in the circulating hemocytes
of the resistant group. Light microscopy revealed five morphological types of circulating cells of both susceptible and resistant snails. Regarding scannig electron microscopy, hemocytes of susceptible snails appeared rounded with smooth or slightly rough surface. However, that of the resistant snails appeared irregular in shaped with corrugated surface. Furthermore, Light microscopy and the transmission electron microscopy revealed signs of cell activation in the hemocytes of the resistant group. The circulating hemocytes consist of five cell types in both susceptible and resistant B. alexandarina and morphologies of these cells are quite similar, but with more signs of cell activations in the resistant group. More specific studies on the functional activities of the hemocytes and mechanisms that may affect or influence the susceptibility and/or non-susceptibility of molluscs to invade microorganisms is essential and how they can act in the immune response.