noun, singular: leucocyte
They are produced and derived from a multipotent cell in the bone marrow, called hematopoietic stem cell. They are called white blood cells because when whole blood is centrifuged, these cells separate into a thin layer that is typically white in color.
There are two types of leucocytes according to the presence of differently staining granules in their cytoplasm. The granulocytes (also known as polymorphonuclear leucocytes) are leucocytes with very distinctive cytoplasmic granules, e.g. neutrophils, basophils and esoniphils. The second is agranulocytes (mononuclear leucocytes), which are characterized by the lack of apparent granules, e.g. lymphocytes, monocytes and macrophages.