White blood cells (WBCs), also known as leukocytes, are fundamental components of the body’s immune system, circulating throughout the bloodstream and lymphatic tissues to defend against infection and disease. Leukemia is a type of cancer originating in the blood-forming tissues, primarily the bone marrow, involving the uncontrolled production and accumulation of abnormal leukocytes. The disease fundamentally disrupts the life cycle and function of these infection-fighting cells. Understanding how leukemia alters white blood cells is central to grasping the pathology of this blood cancer.
The Normal Function of White Blood Cells
Healthy white blood cells originate from hematopoietic stem cells residing within the bone marrow, undergoing a complex maturation process before entering the circulation. Their collective purpose is to recognize, neutralize, and destroy foreign invaders like bacteria, viruses, and fungi. The body continuously produces these cells to maintain a constant supply, as many have short lifespans.
There are five primary types of leukocytes, each with specialized roles in immune defense:
- Neutrophils
- Lymphocytes
- Monocytes
- Eosinophils
- Basophils
Neutrophils are typically the most numerous, acting as first responders to bacterial infections. Lymphocytes are responsible for targeted immunity and producing antibodies.
The Cellular Pathology of Leukemia
Leukemia begins when a genetic mutation occurs in an early blood stem cell, leading to the malignant transformation of its descendants. This mutation typically confers two distinct and damaging characteristics to the developing white blood cells. The first characteristic is uncontrolled proliferation, causing the cancerous cells to multiply rapidly and without the regulatory signals that typically govern cell division.
The second characteristic is impaired maturation, meaning the cells fail to develop into functional, mature white blood cells. These defective, immature cells are often referred to as “blast cells,” and they are unable to perform their intended immune functions. They remain trapped in an early, dysfunctional state while continuing to divide uncontrollably.
The relentless expansion of these non-functional blast cells leads to the physical crowding of the bone marrow. This infiltration displaces and suppresses the production of healthy blood components, including normal white blood cells, red blood cells, and platelets. The resulting shortages lead to common leukemia symptoms, such as an increased risk of infection, anemia, and bleeding problems. This process represents a failure of the white blood cell system, where the sheer number of cells is high, but the number of functional cells is dangerously low.
Categorizing Leukemia by Affected Cell Line
Leukemia is classified according to the speed of its progression and the specific type of white blood cell that undergoes malignant transformation. The cell of origin determines whether the disease is classified as a lymphoid or a myeloid leukemia. Lymphoid leukemias involve the lymphoblast and lymphocyte cell line, which produce T-cells and B-cells.
Myeloid leukemias affect the myeloid cell line, which normally gives rise to granulocytes (like neutrophils), monocytes, red blood cells, and platelets. The names of the four main types—Acute Lymphoblastic Leukemia (ALL), Chronic Lymphocytic Leukemia (CLL), Acute Myeloid Leukemia (AML), and Chronic Myeloid Leukemia (CML)—reflect this distinction. This classification helps predict the disease’s behavior and select appropriate treatment strategies.
The disease is further categorized as either acute or chronic, based on the maturity of the cancerous white blood cells and the speed of progression. Acute leukemias are characterized by a rapid onset and the production of highly immature, non-functional blast cells. Chronic leukemias involve cells that are generally more mature, though still dysfunctional, and the disease progresses at a slower rate.
WBC Counts in Clinical Diagnosis
A Complete Blood Count (CBC) test is the primary tool used to identify initial changes in the blood indicative of leukemia. This test measures the number of red blood cells, platelets, and white blood cells (leukocytes) in the circulation. A WBC count outside the normal range (typically 4,500 to 11,000 cells per microliter) often prompts further investigation.
In many cases of leukemia, the CBC reveals an extremely high white blood cell count, sometimes exceeding 100,000 cells per microliter, due to the uncontrolled proliferation of abnormal cells. However, some patients may present with a normal or even low total WBC count if the cancerous cells are confined to the bone marrow or the marrow is severely suppressed. The most telling finding in acute leukemia is the presence of a high percentage of blast cells in the peripheral blood or bone marrow.
The presence of blast cells, which are normally absent from the bloodstream, signifies an arrest in the normal maturation process. A bone marrow biopsy is often required to confirm the diagnosis. A finding of 20% or more blast cells in the bone marrow typically confirms acute leukemia. Monitoring the blast cell percentage during treatment is a standard method for assessing therapy effectiveness.