Basophils are a type of leukocyte, usually round in shape and 10-12 microns in diameter. They are called basophils because the cytoplasm contains a small number of granules bound to basophilic substances, which often cover the surface of the nucleus. Basophils normally account for 0%-1% of white blood cells. Basophilia can be seen in various allergic diseases, such as allergic enteritis, allergic reactions to inhalants, and rheumatoid arthritis, all of which can cause basophilia. Basophilia can also occur in many hematologic disorders, such as chronic granulocytic leukemia, basophilic leukemia, and myelofibrosis, all of which can result in increased basophil counts. In chronic granulocytic leukemia, a progressive increase in basophils often means that the disease is progressing.