Monocyte

Monocytes are a type of white blood cell, part of the human body's immune system. They are usually identified in stained smears by their large two-lobed nuclei. They are a kind of reserve cell which turn into macrophages and immune helper cells called dendritic cells.
Monocytes work at two speeds in the immune system: 
Half of all monocytes are stored as a reserve in the spleen; the rest are circulating or in tissues. 
Physiology.
Monocytes are produced by the bone marrow from stem cell precursors called "monoblasts". Monocytes circulate in the bloodstream for about one to three days and then typically move into tissues throughout the body. They make up three to eight percent of the leukocytes in the blood. 
Monocytes which migrate from the bloodstream to the tissues will then differentiate into macrophages or dendritic cells, which then stay in the tissue. Macrophages are responsible for protecting tissues from foreign substances. They are cells that possess a large smooth nucleus, a large area of cytoplasm and many internal vesicles for processing foreign material.
Monocytes and their macrophage and dendritic cell progeny serve three main functions in the immune system. These are phagocytosis, antigen presentation and cytokine production.