Leukocytes, or WBCs, in the blood of healthy mammals include segmented neutrophils, band neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Abnormal leukocytes include neutrophils less mature than bands (eg, metamyelocytes, myelocytes, progranulocytes), blast cells of any hematopoietic lineage, mast cells, and other neoplastic cells. WBCs vary in their site of production, their duration of circulation, and the stimuli that affect their release into and migration out of the vascular system. Differences in leukocyte physiology account for species differences in normal blood cell concentrations and their responses in disease.
The leukogram, a component of the CBC, is an organized tabulation of the total nucleated cell concentration, along with the concentrations of specific WBC types present in the sample, also known as the differential. Knowledge of species-specific WBC physiology and the influence of disease processes form the basis for interpreting abnormalities of the leukogram into diagnostic information. Most leukogram interpretations define a process rather than a specific diagnosis. Processes generally fall into four groups: 1) transient physiologic responses that alter vascular hemodynamics; 2) inflammatory, infectious, and immunologic responses; 3) bone marrow responses to injury; and 4) hematopoietic cell neoplasia. (Also see White Blood Cells.)