Many variations in morphology may be seen when examining Wright's stained peripheral blood smears. One method of classifying these variations in white cell morphology is based on the way the body responds to a stimulus, deficiency, or the presence of an inherited defect. This classification falls into three groups:

• Pathological:
  
  Cells may show abnormalities in appearance and/or function. The body is responding abnormally to a stimulus or inherited defect, resulting in physiological impairment in the patient.
  
  
• Nonpathological:
  
  Cells may show variation in morphology but their function is normal. Their presence does not cause physiological impairment.
  
  
• Reactive:
  
  Cells show variation in morphology but are functioning normally in response to a specific stimulus, such as a virus or bacteria. There is a disease process in progress to which the cells are responding. Although the morphology has varied from normal and their presence is significant, the body is responding normally to a stimulus.