Monocytes are the largest type of white blood cell and serve as a critical bridge between the innate and adaptive immune systems. In circulation, they act as scavengers — patrolling for pathogens and damaged cells. When they migrate into tissues, they differentiate into macrophages and dendritic cells, where they process and present antigens to lymphocytes.

Elevated monocytes (monocytosis) are seen in chronic infections (tuberculosis, endocarditis), inflammatory bowel disease, autoimmune conditions, and certain blood cancers including chronic monocytic leukaemia. Low monocytes (monocytopenia) can occur with certain medications or bone marrow suppression. Monocyte count is reported as part of the white blood cell differential on a full blood count.