Final answer:
A left shift in the WBC differential signifies an increase in the number of immature (band) neutrophils, indicating the body's increased production to combat infection or inflammation.
Step-by-step explanation:
When the WBC differential is described as having a left shift, it means there is an increase in the number of immature (band) neutrophils. This occurs as a response to infection, inflammation, or other stressors, indicating that the body is increasing the production of neutrophils to fight off the condition. The presence of more immature neutrophils, which have a distinct single “band” shaped nucleus, signals that there is a demand for these cells beyond what the bone marrow can provide in its normal production of mature neutrophils.
A left shift typically denotes a higher number of band neutrophils and is indicative of an active process in the body where neutrophils are rapidly being used and the bone marrow is attempting to keep up by releasing immature cells into the bloodstream. The neutrophil-lymphocyte ratio may also shift in favor of neutrophils, but that is not what is referred to as a left shift. An increase in hypersegmented neutrophils or a marked decrease in the number of neutrophils may represent other types of hematologic responses, but they are not indicative of a left shift.