What is measured by a differential white blood cell count?

The differential white blood cell count is a laboratory test that specifically measures the percentage and types of different white blood cells (WBCs) present in the blood. This count includes various types of leukocytes such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils, which play critical roles in the immune response. By assessing the proportions of these cells, clinicians can gain insights into an individual’s immune status and potentially diagnose various conditions, including infections, allergies, and hematological disorders.

This ability to differentiate and quantify various WBC types is crucial in understanding the body's response to disease and can indicate specific pathologies based on the changes in the proportions of these cells. For example, an increase in neutrophils might suggest a bacterial infection, while an increase in eosinophils could indicate an allergic reaction.

The other options do not pertain to what a differential white blood cell count measures. The overall number of red blood cells, total blood volume, and concentration of immune cells only do not provide the essential information that a differential count offers regarding the different types of white blood cells and their relative percentages.