Capping of ricin-binding sites does not influence phagocytosis in human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMN'S) were incubated at 0 degrees C with ferritin-conjugated ricin and warmed to 37 degrees C to induce capping of the ricin-binding sites. The PMNs were then allowed to phagocytose yeast or Staphylococcus epidermidis for 15 min, and processed for electron microscopy. Phagocytic uptake, granule fusion, and the fate of lectin-bound membrane were quantified by morphometry. Ricin-capped PMNs phagocytosed as extensively as untreated PMNs. Particles were ingested almost exclusively with a lectin-free portion of the plasmalemma. Fusion of granules with phagocytic vacuoles was not affected by ricin-induced capping. This indicates that ricin-binding sites are not involved in particle recognition and uptake.