Internal defenses of the snail Biomphalaria glabrata.

The three populations of hemocytes and the fixed phagocytes of Biomphalaria glabrata show different affinities for the following injected foreign materials: ferritin, horseradish peroxidase, zymosan particles, latex beads, and live bacteria. Ferritin and horseradish peroxidase, which are internalized by micropinocytosis, induce a moderate activation of hemocytes. Particles trigger a strong activation that consists of mobilization of the cytoskeleton, with formation of pseudopodia and filopodia and modifications of the plasma membrane that enable hemocytes to adhere together, as in encapsulations. Phagocytosis of zymosan and live bacteria induces alterations in mitochondria that may reflect changes in the respiratory activity. Phagocytosis of particles follows different processes: fixed phagocytes ingest latex beads of two different sizes according to the "zipper model", which implies sequential binding between phagocyte receptors and opsonized ligands; zymosan particles are ingested by medium-size hemocytes by a variant of the zipper model in which a limited close contact between cell and particle resembles focal contacts in cell spreading; bacteria or large latex beads are ingested by large hemocytes without apparent close contact following a "trigger model" which resembles macropinocytosis.