monocyte

Monocytes are a type of white blood cell and a key component of the body's innate immune system. They originate in the bone marrow from hematopoietic stem cells through a myeloid lineage pathway, developing first as monoblasts and promonocytes before maturing into circulating monocytes. In humans, monocytes comprise about 2–8% of total leukocytes and typically circulate in the bloodstream for about one to three days before migrating into tissues, where they differentiate into macrophages or dendritic cells.

In the bloodstream, monocytes are relatively large cells with a kidney-shaped nucleus and a variety of surface receptors. They participate in phagocytosis, engulfing pathogens, dead cells, and debris, and they secrete a range of cytokines such as TNF, IL-1, and IL-6 to orchestrate inflammatory responses. Monocytes also function as antigen-presenting cells; after processing antigens, they can present them on MHC class II molecules to T cells, helping to bridge innate and adaptive immunity.

Monocytes are traditionally classified into three subsets based on surface markers and function: classical (CD14++ CD16−), intermediate (CD14++ CD16+), and non-classical (CD14low CD16+). In mice, analogous populations are described using markers such as Ly6C. Monocytes that migrate into tissues differentiate into macrophages or dendritic cells, contributing to tissue repair, wound healing, and responses to infection. Elevated monocyte counts can occur in infection, chronic inflammation, and stress, while low counts may be seen in certain bone marrow disorders or severe illnesses. Monocytes also play roles in atherosclerosis, where they accumulate in vessel walls and can form foam cells.