= Monocyte
A \b[monocyte] is a circulating <leukocyte> of the mononuclear phagocyte system. <Monocytes> participate in recognition, uptake and signaling during immune responses and can contribute cells that differentiate into tissue <macrophages>. The circulating <monocyte> and the tissue <macrophage> are related cell categories, not interchangeable names for every cell in either category.https://meshb.nlm.nih.gov/record/ui?ui=D009000[NLM Monocytes scope note and monocyte/macrophage hierarchy]{ref}
A cell name need not imply a uniform population. In 1989, <B. Passlick> and colleagues used two-color immunofluorescence and <flow cytometry> to identify human blood cells coexpressing <CD14> and <CD16>. The cells were assigned to the <monocyte> lineage using morphology, other markers and functional properties. Their abundance was about 13 percent of the <CD14>-defined <monocytes> in the reported samples, not 13 percent of all blood cells.<Identification and characterization of a novel monocyte subpopulation in human peripheral blood by B. Passlick and others>[Abstract: marker combination, lineage evidence, denominator and functional comparison.]{ref}
For an illustrative count, a sample with 1,000 <monocytes>, of which 130 meet a specified subset definition, has a subset fraction $130/1000=13\%$. If the same tube contains 10,000 <leukocytes>, those 130 cells are only 1.3 percent of <leukocytes>. Changing the denominator changes the biological statement.
The originating comparison also found differences in adherence and uptake of antibody-coated <red blood cells>. A <monocyte> marker panel therefore identifies populations for comparison; it does not guarantee that all identified cells have the same <phagocytosis> rate or response to stimulation.<Identification and characterization of a novel monocyte subpopulation in human peripheral blood by B. Passlick and others>[Abstract: adherence and antibody-coated erythrocyte phagocytosis comparisons.]{ref}
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