A blast cell is an immature, undifferentiated cell that has not yet fully developed to perform a specific function. These cells represent an early stage in a cell’s life cycle, and all specialized cells in the body originate from them. While blast cells are a normal part of human biology, their presence and quantity can provide important information about a person’s health, particularly concerning the blood and bone marrow.
Defining Blast Cells and Their Normal Role
Every type of blood cell begins in the bone marrow through a process called hematopoiesis. This process starts with hematopoietic stem cells, which can develop into any blood cell the body needs. These stem cells first mature into blast cells, also called precursor cells.
Blast cells are an intermediate step, committed to becoming a specific type of blood cell but not yet fully mature. For instance, myeloblasts are destined to become white blood cells, red blood cells, or platelets. Lymphoblasts mature into different white blood cells called lymphocytes (B cells and T cells), which are part of the immune system.
In a healthy person, this production is tightly regulated, and blast cells are found almost exclusively within the bone marrow. They make up less than 5% of the marrow’s cellular population. Under normal circumstances, only fully developed blood cells are released into the bloodstream to do their jobs.
The Link Between Blast Cells and Cancer
The link between blast cells and cancer, particularly acute leukemia, arises when the normal maturation process is disrupted. This is caused by genetic mutations within a blast cell, which gets stuck in its immature state. This defective cell then begins to divide uncontrollably, leading to an overproduction of abnormal, non-functional blast cells.
These cancerous blasts, or leukemic blasts, accumulate rapidly within the bone marrow. As their numbers swell, they crowd out the healthy cells responsible for producing necessary blood components. This “crowding out” effect triggers the primary symptoms of acute leukemia.
A shortage of healthy red blood cells can cause anemia, leading to fatigue and weakness. Fewer functional white blood cells impairs the body’s ability to fight infections. A drop in platelet production leads to easy bruising and bleeding. Eventually, the bone marrow becomes so full of malignant blasts that they spill into the peripheral blood.
Methods for Detecting and Counting Blast Cells
Identifying and quantifying blast cells is a primary step in diagnosing certain blood cancers. The process often begins with a complete blood count (CBC) that reveals abnormal blood cell levels. If leukemia is suspected, a peripheral blood smear is performed by spreading a drop of blood on a glass slide, staining it, and examining it under a microscope for blast cells.
While a blood smear can suggest a diagnosis, a bone marrow examination is the definitive test. This procedure involves using a needle to draw out a liquid sample (aspiration) and a small, solid core (biopsy) of bone marrow, usually from the hip bone. This provides a direct view of blood production and allows for an accurate count of blast cells.
To refine the diagnosis, flow cytometry is used on the bone marrow or blood sample. This method uses lasers to analyze protein markers on the surface of cells. Flow cytometry can precisely identify and count blast cells, confirm they are cancerous, and help classify the exact type of leukemia to determine the best treatment.
Interpreting Blast Cell Counts
The percentage of blast cells found in tests is used to diagnose and understand the severity of the disease. According to the World Health Organization (WHO), a diagnosis of acute leukemia is made when blast cells constitute 20% or more of the cells in the bone marrow or peripheral blood. This threshold distinguishes acute leukemia from less aggressive conditions with elevated blast counts below the 20% mark.
This percentage also serves as a baseline to measure treatment effectiveness. During therapy, physicians order repeat bone marrow biopsies to monitor the blast cell count. The goal of initial treatment is to destroy the cancerous cells and reduce the blast percentage in the bone marrow to a normal level, which is below 5%.
Achieving this state is referred to as remission. Continued monitoring of blast cell counts helps doctors detect any remaining cancer cells that could lead to a relapse. This makes the blast cell count a diagnostic marker and a tool for guiding treatment.