CD14 (Monocytic Cell Marker)

CD14 is a pattern-recognition receptor predominantly expressed on monocytes and macrophages, and to a lesser extent on neutrophils and some dendritic cells. Present as membrane-bound CD14 (mCD14) and soluble CD14 (sCD14) in plasma, it mediates recognition of bacterial lipopolysaccharide (LPS) and is used as a marker of monocyte lineage and innate immune activation in clinical and research settings.

CD14-positive monocytes are circulating innate immune cells that express the CD14 receptor, a co-receptor for recognizing bacterial lipopolysaccharide. They perform phagocytosis, produce cytokines, and can differentiate into macrophages or dendritic cells. Subsets (classical, intermediate, non-classical) vary by CD14/CD16 levels and function. Their numbers and activation rise during infection and inflammation and are used as clinical immune markers.

CD14 is the most widely used and best single surface marker for human blood monocytes. Combined CD14 with CD16 differentiates classical (CD14++CD16−), intermediate (CD14++CD16+), and non-classical (CD14+CD16++) subsets. CD68 and HLA-DR are additional myeloid/macrophage markers but are less specific for circulating monocytes. Flow cytometry using these markers is the standard method to identify and quantify monocyte populations.

CD14 monocytes (CD14++CD16−) are the predominant subset, with high CD14 expression, strong phagocytic activity, LPS responsiveness and capacity to become tissue macrophages or dendritic cells. CD16 monocytes (CD14+CD16+ or CD14dimCD16++) are fewer, more pro‑inflammatory and “patrolling,” producing TNF-α/IL‑1β, surveilling endothelium and responding to viral or tissue-injury signals.

CD14 is also known as cluster of differentiation 14 or monocyte differentiation antigen CD14. It functions as an LPS (lipopolysaccharide) co‑receptor on monocytes and macrophages. CD14 exists as membrane-bound CD14 (mCD14) and a circulating soluble form (sCD14), both involved in innate immune recognition of bacterial components. It is a myeloid cell surface marker aiding TLR4/MD-2 signalling to trigger inflammatory responses.

The classic human monocyte marker is high CD14 expression (CD14++), with absence of CD16 — commonly denoted CD14++CD16−. These classical monocytes also express CCR2 and comprise the majority of circulating monocytes, mediating inflammatory recruitment and phagocytosis. They are CD45+ leukocytes.