A monocyte is a leukocyte, part of the human body's immune system that protect against blood-borne pathogens and move quickly to sites of infection in the tissues. It is one of the five major types of white blood cell, based on the appearance of white blood cells in stained smears as viewed under a light microscope.
Monocytes are produced by the bone marrow from haematopoietic stem cell precursors, circulate in the blood stream for about one to three days and then typically move into tissues throughout the body. In the tissues monocytes mature into different types of macrophages at different anatomical locations.
Monocytes are responsible for phagocytosis (ingestion) of foreign substances in the body. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition receptors that recognize pathogens. Monocytes are also capable of killing infected host cells via antibody, termed antibody-mediated cellular cytotoxicity. Vacuolization may be present in a cell that has recently phagocytized foreign matter.
Monocytes which migrate from the blood stream to other tissues are called macrophages. Macrophages are responsible for protecting tissues from foreign substances but are also the predominant cells involved in atherosclerosis. They are cells that possess a large smooth nucleus and a large area of cytoplasm.
A monocyte count is part of a complete blood count and is expressed either as a ratio of monocytes to the total number of white blood cells counted, or by absolute numbers. Both may be useful. Monocytosis is the state of excess monocytes in the peripheral blood. It may be indicative of various disease states.