White blood cells (WBCs) are the foot soldiers of the immune system. They help fight infections, limit injury from other harmful substances, and initiate healing if tissue damage occurs. WBCs are made and primarily stored in the bone marrow. Only a small percentage normally circulate in the blood, but stored WBCs can be rapidly released into the bloodstream if needed. A high white blood cell count is usually related to an immune system response, commonly due to an infection or inflammation in the body. Less commonly, a high WBC count is due to overproduction by the bone marrow.
Bacterial and viral infections are the most common causes of a high white blood cell count. Fungal or parasitic infections can also increase the number of white blood cells in the blood. The increase in WBCs due to infection is caused by a normal immune system response that stimulates the bone marrow to release stored white blood cells. They travel in the bloodstream to the site of the infection to fight the invading germs. With an infectious cause, the white blood cell count returns to normal once the infection clears.
Inflammation and Allergy
Conditions that cause inflammation in the body can lead to a high white blood cell count. This includes medical conditions such as rheumatoid arthritis, ulcerative colitis and Crohn disease. Tissue damage and injury -- such as burns, a heart attack and surgery -- can also cause inflammation. Inflammation triggers chemical messages that cause stored WBCs from the bone marrow to be released in increased numbers. Severe allergies and asthma are other forms of inflammation that may increase the white blood cell count.
Stress, Drug Reactions and Smoking
Severe physical or emotional stress can also cause an increased WBC count, including stress caused by overexertion and anxiety. Based on a 2012 research review published in the journal "Exercise Immunology Review," exercise and stress can increase levels of the adrenal hormone cortisol and other hormones. These hormones are thought to be responsible for recruiting WBCs into the bloodstream.
Some medications, including certain drugs used to treat inflammation, mental health conditions and cancer, can also lead to increased WBC release into the blood. For example, a study in the December 2001 issue of the "British Journal of Haematology" suggests that lithium -- a drug used to treat bipolar disorder -- signals the immune system to produce more WBCs. Smoking can also affect the bone marrow and cause increased release of certain types of WBCs, potentially increasing the white blood cell count.
Bone Marrow Diseases
A bone marrow disorder is a less common, more serious cause of elevated white blood cells. Leukemia is the most frequent culprit. With this type of cancer, the bone marrow produces large numbers of defective white blood cells and releases them into the blood. Other bone marrow disorders, such as polycythemia vera, can also increase the WBC count. Polycythemia vera is a disease in which too many red blood cells are produced. An excess of WBCs might also be produced with this condition. Myelofibrosis is a disorder in which the bone marrow is replaced with scar-like tissue. This can cause abnormal production of blood cells leading to an increased WBC count during the initial, inflammatory phase of the disease.