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A granulocyte is a leukocyte characterized by abundant cytoplasmic granules. The major circulating categories include neutrophils, eosinophils and basophils. The term groups cells by characteristic morphology and related biology; it does not mean that every granule has the same contents or that every granulocyte performs the same immune function.^[ref]

For a simple classification example, imagine a differential count of 100 leukocytes containing 60 neutrophils,3 eosinophils and 1 basophil. The total is 64 granulocytes, not 60, even though neutrophils are the largest component in this invented sample. The example separates a broad category from its most numerous member; it is not a reference interval for a patient.

Morphology also should not be substituted for a functional measurement. S. D. Wright and S. C. Silverstein measured hydrogen peroxide release when human granulocytes and other phagocytes spread on surfaces bearing different recognition molecules. Surfaces coated with IgG produced a strong oxidant response, whereas C3b- or iC3b-coated surfaces did not produce appreciable peroxide release under the tested conditions.^[ref]

Thus the presence of granules or a capacity for phagocytosis does not by itself establish which signaling pathway was activated. Cell identity, receptor stimulation, uptake and extracellular release are different experimental variables.

A phagocyte is a biological cell that takes up particles, such as bacteria or cell debris, by phagocytosis: the cell membrane encloses the material in an intracellular phagosome. Macrophages and neutrophils are important examples. The term describes an uptake capability, rather than a single developmental lineage. Contact with a particle, internalization and destruction of the ingested material are distinct events.

A useful experimental distinction is between phagocytosis and production of reactive oxygen compounds. S. D. Wright and S. C. Silverstein measured hydrogen peroxide released by human granulocytes, monocytes and macrophages spreading on surfaces coated with defined recognition molecules. Surfaces carrying IgG stimulated peroxide release, whereas surfaces carrying C3b or iC3b did not appreciably do so. Even when fibronectin supported a strong complement-receptor-associated phagocytic response in cultured monocytes, measurable peroxide release did not follow.^[ref]

For example, counting ten particle-containing phagosomes would establish uptake in the observed phagocyte; it would not count ten killed bacteria. A killing assay needs a further endpoint, such as recovery of viable organisms. Likewise, a low extracellular peroxide measurement alone does not establish absence of phagocytosis. Choosing an endpoint that matches the biological question is essential when comparing phagocytes.

S. D. Wright investigated how receptors of phagocytes connect recognition of coated targets with distinct cellular responses. With S. C. Silverstein, Wright compared hydrogen peroxide release from granulocytes, monocytes and macrophages spreading on surfaces coated with different ligands. The 1983 study found strong release on IgG-coated surfaces but little on surfaces carrying C3b or iC3b, historically written C3bi.^[ref]

The useful mechanistic distinction was between engulfment and an oxidative response. Under conditions combining fibronectin with complement ligands, the cultured monocytes showed a strong phagocytosis response without measurable hydrogen peroxide release.^[ref] Thus receptor engagement does not specify one indivisible activation state. Measuring only uptake would miss a difference revealed by measuring a second output.