The white blood cell differential assesses the ability of the body to respond to and fight infection. It also detects the severity of allergic reactions, parasitic and other types of infection, and drug reactions. It can also identify some types of leukaemia or lymphoma.

The white blood cell differential is normally run as part of the full blood count (FBC), which is requested for many different conditions.

The results indicate the percentage and absolute number of each type of white blood cell that is present.

• Neutrophils can increase in response to bacterial infection, inflammatory disease, steroid medication, or more rarely leukaemia. Decreased neutrophil levels may be the result of severe infection or other conditions, such as responses to various medications or chemotherapy.
• Lymphocytes can increase in cases of bacterial or viral infection, leukaemia, lymphoma, radiation therapy or acute illness. Decreased lymphocyte levels are common in later life but can also indicate steroid medication, stress, lupus and HIV infection.
• Monocyte levels can increase, in response to infection of all kinds as well as to inflammatory disorders. Occasionally persistently elevated monocyte levels may be associated with some types of leukaemias. Decreased monocyte levels can indicate bone marrow injury or failure and some forms of leukaemia.
• Eosinophils can increase in response to allergic disorders, inflammation of the skin and parasitic infections. They can also occur in response to some infections or to various bone marrow malignancies.
• Basophils can increase in cases of leukaemia, long-standing inflammation, the presence of a hypersensitivity reaction to food, or radiation therapy.
• Immature forms may also be present in certain conditions. These include metamyelocytes, myelocytes, promyelocytes and blast cells.

Eating, physical activity and stress may alter white blood cell differential values.

Long-term exposure to toxic chemicals (for example some solvents, petroleum products and insecticides) can increase the risk of an abnormal differential.

• Neutrophils, the most abundant white blood cells, are 'phagocytes' - that is, they 'eat' foreign organisms and kill them with internal poisons. They are important for fighting bacterial and fungal infections.
• Lymphocytes are more specifically acting killers of infection and regulate the immune response. B-lymphocytes produce antibodies. T-cells act as messenger cells, directing the immune response. T-cells and NK-cells can act directly to kill specific infective organisms. Lymphocytes are the cells that 'remember' previous infections and guard against reinfection with the same organism. They are active against all types of infection including bacteria, fungi, parasites and viruses. When they malfunction they also have a role in allergy and in autoimmune diseases such as rheumatoid arthritis and lupus.
• Monocytes are phagocytes, like neutrophils, active against bacteria, fungi, and tuberculosis.
• Eosinophils increase during allergic attacks and some parasitic infestations.
• Basophils control inflammation and damage to the body. They increase in some blood diseases and poisoning.