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High‐Resolution Localization of Lactoferrin in Human Neutrophils: Labeling of Secondary Granules and Cell Heterogeneity

We studied by electron microscopy the topography of the iron‐binding protein lactoferrin in human peripheral blood neutrophils by immunogold labeling of thin‐sectioned cells. We show that fixation in 0.3% glutararaldehyde, supplemented by 2% formaldehyde, and combined with embedding in the hydrophilic resin LR White results in excellent preservation of the lactoferrin antigenicity and of the neutrophil microanatomy. Most of the immunogold marking was seen on the specific (secondary) granules of polymorphonuclear leukocytes, thus strengthening prevailing views on the subcellular distribution of lactoferrin in neutrophils. We document that a minority (8‐10%) of neutrophils showed significant amounts of lactoferrin associated with a flocculent, protein‐like material seen inside large cytoplasmic vacuoles. We show that circulating neutrophils comprise 6‐10% of cells that are virtually devoid of lactoferrin labeling. This finding provides specific cytochemical support for the concept that human polimorphonuclear leukocytes are made up of a heterogeneous population of cells.