The blood differential test measures the percentage of each type of white blood cell (WBC) that you have in your blood. It also reveals if there are any abnormal or immature cells.
Five types of white blood cells, also called leukocytes, normally appear in the blood:
Neutrophils
Lymphocytes
Monocytes
Eosinophils
Basophils

A special machine or a health care provider counts the number of each type of cell. The test shows if the number of cells are in proper proportion with one another, and if there is more or less of one cell type.
What Abnormal Results Mean
Any infection or acute stress increases your number of white blood cells. High white blood cell counts may be due to inflammation, an immune response, or blood diseases such as leukemia.
It is important to realize that an abnormal increase in one type of white blood cell can cause a decrease in the percentage of other types of white blood cells.

Acute infection
Acute stress
Eclampsia (seizures or coma in a pregnant woman)
Gout (type of arthritis due to uric acid buildup in the blood)
Acute or chronic forms of leukemia
Myeloproliferative diseases
Rheumatoid arthritis
Rheumatic fever (disease due to an infection with group A streptococcus bacteria)
Thyroiditis (a thyroid disease)
Trauma
Cigarette smoking
Aplastic anemia
Chemotherapy
Influenza (flu)
Radiation therapy or exposure
Viral infection
Widespread severe bacterial infection
Chronic bacterial infection
Infectious hepatitis (liver swelling and inflammation from bacteria or viruses)
Infectious mononucleosis, or mono (viral infection that causes fever, sore throat, and swollen lymph glands)
Lymphocytic leukemia (a type of blood cancer)
Multiple myeloma (a type of blood cancer)
Viral infection (such as mumps or measles)
Chemotherapy
HIV/AIDS infection
Leukemia
Radiation therapy or exposure
Sepsis (severe, inflammatory response to bacteria or other germs)
Steroid use
Chronic inflammatory disease
Leukemia
Parasitic infection
Tuberculosis, or TB (bacterial infection that involves the lungs)
Viral infection (for example, infectious mononucleosis, mumps, measles)
Addison disease (adrenal glands do not produce enough hormones)
Allergic reaction
Cancer
Chronic myelogenous leukemia
Collagen vascular disease
Hypereosinophilic syndromes
Parasitic infection
After splenectomy
Allergic reaction
Chronic myelogenous leukemia (a type of bone marrow cancer)
Collagen vascular disease
Myeloproliferative diseases (group of bone marrow diseases)
Chickenpox
Acute infection
Cancer
Severe injury