Some diseases unrelated to a malignant blood dyscrasia may occasionally be associated with a blood leukocyte response which resembles leukemia closely enough to cause the physician concern about the diagnosis. This blood picture may be termed leukemoid either because of a striking increase in blood leukocyte concentration or because of the presence in the blood of immature cells found normally only in the bone marrow. A leukemoid reaction can be classified as lymphocytic, monocytic, or granulocytic according to the type of cell involved. In the following discussion only leukemoid reactions of the neutrophilic myeloid cells will be considered.
The term "leukemoid" was first used by Krumbhaar 1 in 1926 to describe a group of nonleukemic conditions which were thought to be indistinguishable from leukemia by blood examination alone. Included in his original series were cases of terminal septicemia, mustard gas poisoning, and a patient with coexisting acute infection and hemorrhage.