Leucocytosis is a medical term defining an elevated number of white blood cells (WBCs), also called leukocytes, circulating in the blood. These cells are produced in the bone marrow and represent the primary line of defense for the body’s immune system. Their function is to identify and neutralize foreign invaders, such as bacteria and viruses, and to clean up damaged tissue. Leucocytosis is not a diagnosis itself; rather, it is a sign indicating the body is actively reacting to stress, inflammation, or infection.
Classifying Leucocytosis by Cell Type
Leucocytosis is categorized based on which of the five main types of white blood cells is primarily responsible for the elevation. This classification provides a clue for pinpointing the underlying cause, as each cell type targets different threats.
Neutrophilia
Neutrophilia is the most common form, representing an increase in neutrophils, the most abundant type of WBC. Neutrophils are rapid-response cells that primarily engulf and destroy bacteria and fungi through phagocytosis. An increase in these cells is the body’s typical immediate reaction to acute bacterial infection or significant tissue damage.
Lymphocytosis
Lymphocytosis refers to an elevated count of lymphocytes (T cells and B cells), which are the core components of the adaptive immune system. These cells recognize specific pathogens and create immunological memory. A rise in lymphocytes is most often associated with viral infections, such as mononucleosis or hepatitis.
Eosinophilia
Eosinophilia is an increase in eosinophils, which specialize in combating larger parasites. Eosinophils are also involved in allergic responses, releasing chemicals that contribute to inflammation. Therefore, a high eosinophil count suggests either a parasitic infection or an allergic condition.
Monocytosis
Monocytosis is the elevation of monocytes, large WBCs that circulate before migrating into tissues to become macrophages. Once in the tissue, these macrophages perform long-term cleanup and immune regulation. An elevated monocyte count often occurs in the context of chronic inflammation, autoimmune diseases, or certain bacterial infections like tuberculosis.
Common Triggers and Underlying Conditions
The production of excess white blood cells is a response to a wide range of stimuli, from routine physical exertion to serious illness. Acute infections are the most frequent cause, where bacteria trigger a massive release of neutrophils from the bone marrow. Conversely, a systemic viral infection typically prompts an increase in lymphocytes.
Inflammatory conditions that do not involve infection, such as rheumatoid arthritis or tissue injury from severe burns, also lead to leucocytosis. This elevation is part of the body’s generalized healing response, mobilizing various WBCs to clear debris and initiate repair. Certain medications, especially corticosteroids, can cause transient leucocytosis by preventing white blood cells from adhering to blood vessel walls, causing more of them to circulate freely.
Physical and emotional stress can also act as triggers, leading to a temporary elevation in the total WBC count. Intense exercise, seizures, or a high-stress event can cause a surge of stress hormones that mobilize WBCs from their storage pools. In contrast to these reactive causes, sustained leucocytosis can signal a primary bone marrow disorder, such as leukemia, where the bone marrow produces an uncontrolled number of immature or dysfunctional white blood cells.
How Leucocytosis is Detected
Leucocytosis is typically detected through a Complete Blood Count (CBC). This common blood test measures the total number of white blood cells, red blood cells, and platelets. For most healthy adults, a total white blood cell count exceeding approximately 11,000 cells per microliter of blood is considered leucocytosis.
The CBC usually includes a differential count, which is a detailed breakdown of the five types of white blood cells. The differential is crucial, as it immediately identifies which cell line is driving the total count elevation. For instance, a high absolute number of neutrophils points toward a bacterial cause, while a high lymphocyte count suggests a viral cause.
While leucocytosis itself may not produce specific symptoms, the underlying condition often does. Fever, fatigue, or localized pain are commonly associated with the infections or inflammation that stimulate the increase in WBCs. The total count and the specific cell profile help guide further investigation, such as additional testing for an infection or a bone marrow evaluation.
Addressing the Root Cause
The management of leucocytosis focuses on identifying and treating the underlying condition that is stimulating the white blood cell increase. The elevated count is a protective, adaptive response, and trying to reduce the count without addressing the trigger is generally not the correct approach. For example, a bacterial infection requires antibiotic therapy to eliminate the pathogen, allowing the WBC count to naturally return to the normal range.
Similarly, if leucocytosis is linked to an autoimmune disorder, treatment centers on managing chronic inflammation through targeted medications. Once the immune response is no longer stimulated by the primary trigger, the bone marrow slows production, and the number of circulating WBCs gradually normalizes.
An exception occurs in rare cases of extreme elevation, known as hyperleukocytosis, where the WBC count can exceed 100,000 cells per microliter. This extreme increase, often seen in specific types of leukemia, can cause the blood to become excessively thick (leukostasis), which is a medical emergency. In these situations, specialized interventions, such as leukapheresis or immediate chemotherapy, are used to rapidly lower the cell count and prevent complications like blood vessel blockage.