Medical Laboratory Scientist (MLS) ASCP Exam 2025 – 400 Free Practice Questions to Pass the Exam

In a leukemoid reaction, the body responds to a stressor, such as severe infection or inflammation, by increasing the production of white blood cells, specifically neutrophils. This results in a significantly elevated white blood cell count, often exceeding 50,000 cells per microliter. In addition to the increase in overall white blood cells, there is typically a left shift observed in the neutrophil lineage, indicating the presence of immature neutrophils (such as band forms and myelocytes) in circulation. This reflects the bone marrow's increased output in response to a heightened demand for neutrophils to fight off the underlying condition.

The combination of a high white blood cell count and the left shift is a hallmark of a leukemoid reaction and serves as an important differentiating feature from other conditions, such as leukemia. In those cases, the structure and profile of the white blood cell count can differ significantly. Thus, the presence of a high white blood cell count with a left shift is characteristic of a leukemoid reaction.