Monocyte white blood cell. Coloured transmission electron micrograph (TEM) of a section through a monocyte white blood cell, showing the cell nucleus (purple), mitochondria (cyan ovals) and characteristic granules (brown). Monocytes, like all white blood cells, are part of the human body's immune system. Macrophages (and their precursors, monocytes) are the 'big eaters' of the immune system. These cells reside in every tissue of the body, where they engulf apoptotic cells and pathogens and produce immune effector molecules.Magnification: x6000 when printed at 10 centimetres wide.