During the course of granulocyte collection by continuous-flow filtration leukopheresis, an abrupt fall in neutrophil count was noted (mean decrease 77%, range 64%-95%). Neutropenia occurred within 5 min of return of blood exposed to the nylon fiber filters and lasted less than 30 min. Saline exposed to the fibers, withdrawal and reinfusion of whole blood, and heparin did not cause neutropenia. Heparinized blood passed by gravity through isolated filters and reinfused immediately also induced neutropenia (mean decrease 64% +/- 8%, range 11%-19%). Blood anticoagulated with ACD (decrease 19.5% +/- 6%, range 6%-56%), heparinized plasma (N = 10, decrease 15% +/- 3%, range 3%-29%) and platelet-rich plasma exposed to the filters failed to produce neutropenia. 91% +/- 2% of the neutrophils adhered to the fibers using heparinized blood as compared to 21% +/- 5% using ACD (p less than 0.001). All donors were asymptomatic during the infusions. These results suggest that during neutrophil adherence a substance is released which produces profound, transient neutropenia perhaps by inducing margination of cells.

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