Leukocytosis describes a condition where a blood test reveals an elevated number of white blood cells (WBCs) circulating in the bloodstream. While leukocytosis can signal a malignancy, most cases are a normal, temporary response to a far less threatening underlying issue. A subsequent medical evaluation is necessary to determine the specific, underlying cause of the increased cell count.
Understanding White Blood Cells and Their Role
White blood cells, or leukocytes, are the body’s dedicated defense system, originating from stem cells within the bone marrow. These cells are constantly circulating, ready to mobilize against foreign invaders or internal damage. The immune response requires a swift and coordinated increase in production and release of these cells when a threat is identified.
The immune system utilizes five primary types of WBCs, each with a distinct specialized function. Neutrophils are typically the most numerous and are the first responders to bacterial infections and inflammation. Lymphocytes, which include T-cells and B-cells, are responsible for targeting viruses, coordinating the long-term immune memory, and producing antibodies.
Monocytes circulate in the blood before migrating into tissues to become macrophages, which act as powerful “cleanup crews.” Eosinophils are primarily involved in the defense against parasitic infections and the mediation of allergic responses. Basophils are the least common type and release histamine, a chemical that plays a role in initiating inflammatory and allergic reactions.
Common Causes That Are Not Cancer
Most leukocytosis cases are a reactive process where the bone marrow responds appropriately to a temporary stimulus. Acute infections, whether bacterial or viral, are the most frequent cause, prompting a rapid spike in WBC production to eliminate the pathogen. Once the infection is cleared, the white blood cell count typically returns to the normal range.
Inflammatory conditions, such as autoimmune diseases like rheumatoid arthritis or inflammatory bowel disease, also trigger a sustained elevation in WBCs. In these scenarios, the body perceives its own tissues as a threat, causing chronic activation of the immune system. Physical stress, including severe trauma, burns, or intense physical exertion, can temporarily increase the count by mobilizing stored cells from the bone marrow and blood vessel walls.
Certain medications are well-known for their side effects on white blood cell counts. Corticosteroids, for example, can cause a significant increase by reducing the WBCs’ ability to move out of the blood and into tissues. Lifestyle factors, such as heavy smoking, can also result in a chronic, low-grade leukocytosis that generally resolves after the habit is discontinued.
Leukocytosis as a Sign of Blood Cancer
When leukocytosis signals cancer, the mechanism differs fundamentally from a normal immune response. In blood cancers like leukemia or myeloproliferative neoplasms (MPNs), the increase results from the uncontrolled, malignant overproduction of abnormal cells in the bone marrow. This proliferation, often involving immature “blast” cells, overwhelms the normal blood cell production lines.
Cancer-related leukocytosis often involves extremely high and persistent cell counts, sometimes exceeding 100,000 cells per microliter, a condition known as hyperleukocytosis. Unlike reactive leukocytosis, the cells produced are often dysfunctional and unable to fight infections effectively. The presence of immature or morphologically abnormal cells is a major distinguishing feature of malignancy.
This type of leukocytosis is frequently accompanied by systemic symptoms. These include unexplained fevers, night sweats, significant fatigue, easy bruising, or bleeding. These clinical features, combined with the extreme cell count, raise suspicion for a primary bone marrow disorder.
Diagnostic Procedures to Identify the Source
Determining the cause of an elevated white blood cell count begins with a complete blood count (CBC) that includes a differential count. The differential measures the percentage and absolute number of each of the five types of WBCs, which provides the first critical clue. For instance, an elevation dominated by neutrophils suggests a bacterial infection, while a lymphocytosis might point toward a viral infection.
Following the initial blood work, a healthcare provider conducts a thorough physical examination and reviews the patient’s medical history, searching for signs of infection, inflammation, or medication use. If malignancy is suspected, the next step is usually a peripheral blood smear. This procedure involves microscopically examining the blood cells for abnormal size, shape, or the presence of highly immature cells, such as blasts.
If the blood smear shows concerning features or if the cell count is extremely high and unexplained, a bone marrow aspiration and biopsy may be necessary. This procedure involves collecting a small sample of the spongy tissue inside the bones where blood cells are produced. Analyzing the bone marrow is the definitive way to confirm or rule out a blood cancer by assessing cellularity, the ratio of different cell types, and the presence of genetic abnormalities.