Unopsonized zymosan uptake and activation of human neutrophils. (A) Neutrophil phagocytosis of zymosan is shown for neutrophils purified from a healthy donor (left panel) and a patient (right panel); in case of patient samples, particles were easily washed off during the preparation of cytospins because of the defective phagocytosis (n = 4 patients). (B) Neutrophils from a healthy control were stimulated with serum-treated zymosan (STZ) or unopsonized zymosan as described in “Patients, materials, and methods” (n > 100 controls). (C) Identical experiments with neutrophils from LAD-1/variant patients demonstrate the lack of NADPH oxidase activity in the absence of phagocytosis of the unopsonized zymosan particles (left bars; n = 12 controls and n = 5 patients [following similar procedures and timing], mean ± SD, at 10 minutes after addition of zymosan). The effect of simultaneous addition of Mg²⁺ and the zymosan particles indicates the potential to react in a CR3-dependent manner (right bars).