Monocytes

Monocytes are a type of white blood cell that forms part of the innate immune system. They develop in the bone marrow from myeloid progenitors and circulate in the bloodstream before migrating into tissues. In humans they comprise a notable fraction of circulating leukocytes and typically reside in the blood for about one to three days before tissue entry.

In tissues, monocytes differentiate into macrophages or dendritic cells, where they play a key role in host defense and tissue homeostasis. Their functions include phagocytosis of pathogens and debris, production of inflammatory and regulatory cytokines, and the presentation of antigens to T cells via MHC class II molecules. Monocytes can also contribute to wound healing and tissue remodeling after injury.

Subsets and markers vary by species. In humans, classical monocytes are characterized by high CD14 expression and lack of CD16, while intermediate (CD14++ CD16+) and non-classical (CD14+ CD16++) subsets have distinctive roles in inflammation and tissue surveillance. In mice, inflammatory monocytes are typically Ly6C high, whereas patrolling monocytes are Ly6C low. Recruitment to sites of inflammation is guided by chemokine signals, notably CCR2 for egress from the bone marrow and CCL2/MCP-1–driven recruitment; patrolling monocytes depend more on CX3CR1 signaling.

Clinical relevance includes monocyte involvement in infections, chronic inflammatory diseases, atherosclerosis, and tumor biology. Abnormal monocyte levels or function can reflect or contribute to disease states, making them a focus of research and diagnostic assessment.