Leukemia is a term that originally was applied to a few cases in which the blood appeared milky at necropsy. Since such condition can arise only when the leukocyte count almost equals that of the red cells, it means that the leukocyte count must be about 1,000,000 per cubic millimeter and the anemia profound. Only one disease can produce it, the one we still call leukemia or cancer of the blood.

However, in the great majority of cases of this disease the blood is red, since the leukocyte count seldom rises above 500,000 per cubic millimeter.

Several varieties of leukemia or blood cancer are recognized. They all are characterized by the presence of immature leukocytes of one type or another in the peripheral blood and by extensive hyperplasia or overgrowth of the tissue producing that particular cell. The most common are granulocytic, lymphocytic, and monocytic leukemia. These terms referring in each case to the cell type that is involved.

The clinical course of leukemia may be fulminating, progressing to a fatal termination usually within a period of a few weeks or months.

Whether acute or chronic, every leukemia process is a manifestation of malignant neoplasia or the uncontrolled, destructive proliferation of a blood cell that has acquired through some genetic alteration of a biologic advantage over other cells. Thus, this condition enabled the other cells to outgrow them, even in their native tissues, their optimal environments, and freeing it from the physiologic restraints that usually prevent overgrowth.