There are 5 types of white cells found in circulating blood: eutrophils, eosinophils, basophils, monocytes, and lymphocytes.
These five can be divided into two groups, granulocytes (also known as polymorphonuclear leukeocytes) and mononuclear leukocytes. Neutrophils, eosinophils, and basophils are granulocytes, and have cytoplasm which has a grainy appearance and they have nuclei that are elongated and have several rounded portions called lobes. The lobed nuclei are the reason these cells are called polymorphonuclear; the lobes are connected only by thin threads, and so it almost looks like many nuclei instead of just one. Mononuclear leukocytes may have nuclei that are indented, but not so deeply divided that they appear to have multiple nuclei. Mononuclear leukocytes are also called "non-granulocytes." Each type of leukocyte, or white blood cell (WBC), has a function in supporting the immune system, and elevated numbers of WBCs in the blood can be one of the first indicators of leukemia.