A high white blood cell count is an increase in cells in the blood that fight infections.

What's considered high in a white blood cell count varies from one lab to another. This is because laboratories set their own reference ranges based on the populations they serve. In general, for adults a count of more than 11,000 white blood cells in a microliter of blood is considered high.

A high white blood cell count usually means one of the following has increased the making of white blood cells:

• An infection.
• Reaction to a medicine.
• A bone marrow disease
• An immune system issue.
• Sudden stress such as hard exercise.
• Smoking.

Specific causes of a high white blood cell count include:

• Allergy, especially severe allergic reactions
• Asthma
• Bacterial, viral, fungal or parasitic infections
• Burns
• Churg-Strauss syndrome
• Medicines, such as corticosteroids and epinephrine
• Hay fever (allergic rhinitis)
• Leukemia
• Lymphoma
• Myelofibrosis (a bone marrow disorder)
• Polycythemia vera
• Pregnancy
• Rheumatoid arthritis
• Sarcoidosis
• Smoking
• Tuberculosis
• Vasculitis
• Whooping cough

A test a health care provider orders to diagnose a condition can reveal a high white blood cell count. A high white blood cell count is rarely found by chance.

Talk to your care provider about what your results mean. A high white blood cell count plus results from other tests might show the cause of your illness. Or you might need other tests for more information about your condition.