Monocytosis is an increase in the number of monocytes circulating in the blood. Monocytes are white blood cells that give rise to macrophages and dendritic cells in the immune system. In humans, monocytosis occurs when there is a sustained rise in monocyte counts greater than 800/mm3 to 1000/mm3. Monocytosis has sometimes been called mononucleosis, but that name is usually reserved specifically for infectious mononucleosis..