Table 1.

Categorisation of monocyte subsets in health.

No of Subsets	Name of Subsets	Identifying Markers	Method Used	Reference	Year
1	Monocytes Monocytes	No markers used No markers used	Immunohistochemistry staining, phagocytosis Cytochemistry, light microscopy, Biochemical assays	[67] [52]	1926 1966
2	Monocyte large fraction (80–90%) Intermediate small fraction (12–18%) Monocytes	Esterase & Fc receptor detection CD14++ CD14+/CD16+	Elutriation. Cell fractions determined by peroxidase and esterase staining, neutral red phagocytosis, and iron particle phagocytosis 2 colour fluorescence flow cytometry & cell sorting	[53] [58]	1979 1989
	Round oval (type a) Slightly folded (type b) Distinctly folded (type c)	Measured Chloroacetate-Esterase, Acetate-Esterase and 3H-TDR labelling index	Cytochemical reactions and DNA-Synthesis activity	[54]	1974
	Monocytes	Detection of Fc & Complement receptors	Cytochemical stains, C3 receptor assays	[55]	1979
3	Classical Intermediate Nonclassical	CD14++CD16− CD14++CD16+ CD14+CD16++	Flow cytometry	[63]	2010
	Classical Intermediate Nonclassical	CD14++CD16− CD14++CD16+ CD14+CD16++	Microarray, flow cytometry and cytokine production	[64]	2011
	Classical Intermediate Nonclassical	CD14++CD16− CD14++CD16+ CD14+CD16++	SuperSAGE transcriptome analysis	[65]	2011
	Classical Intermediate Nonclassical	CD14++CD16− CD14++CD16+ CD14+CD16++	CyTOF mass cytometry	[66]	2017
4	Mono1 (classical) Mono3 Intermediate) Mono4 (intermediate) Mono2 (nonclassical)	CD14++CD16− CD14++CD16+ * CD14+CD16++	Single-cell RNA sequencing (scRNA-seq)	[68]	2017

* Within this subset, single-cell RNA-seq identified heterogeneity, namely two distinct clusters which were assigned Mono3 and Mono4. Gene expression profiling showed Mono3 expressed cell cycle and trafficking genes, while Mono4 expressed a cytotoxic gene signature.