Flow cytometry is a laboratory technique used to analyze cells or other particles. While commonly used for blood samples, it is not exclusively a blood test. This versatile method provides detailed information about individual cells, making it a powerful tool in various scientific and medical fields. It goes beyond simple cell counting to provide a comprehensive profile of cell populations.
Understanding Flow Cytometry
Flow cytometry rapidly analyzes individual cells or particles as they pass in a single file through a laser beam. This process allows for the measurement of various physical and chemical characteristics of each cell. The technique can assess a cell’s size and internal complexity, known as granularity. Forward scatter (FSC) measurements relate to cell volume, while side scatter (SSC) provides insights into the internal structure and granularity.
Beyond physical properties, flow cytometry also measures the presence and amount of specific molecules on or within cells. This is achieved by labeling cells with fluorescent markers, often attached to antibodies that bind to particular cellular components. When the laser beam excites these fluorescent markers, they emit light at specific wavelengths, which is then detected and quantified. This ability to simultaneously analyze multiple parameters on thousands of cells per second provides a detailed profile of heterogeneous cell populations.
Samples Analyzed by Flow Cytometry
Blood is a common sample type for flow cytometry analysis, frequently employed to study immune cells or detect abnormal cells in conditions like leukemia and lymphoma. However, flow cytometry is not limited to blood samples. Many other biological fluids and tissues can also be analyzed.
Bone marrow is another frequently analyzed sample, particularly in the diagnosis and monitoring of blood cancers. Cerebrospinal fluid (CSF), which surrounds the brain and spinal cord, can also be examined for conditions affecting the central nervous system. Tissue biopsies, after being processed into a single-cell suspension, can also be used to investigate cellular characteristics within solid organs. The choice of sample type depends on the specific medical question or research objective.
When Flow Cytometry is Used
Flow cytometry is widely used in clinical settings for diagnosing and monitoring various diseases, especially those involving the immune system and blood cells. A significant application is in the diagnosis and classification of blood cancers such as leukemias and lymphomas. It helps identify the specific type of cancer cells based on unique protein markers on their surface, guiding treatment decisions.
The technique is also instrumental in monitoring minimal residual disease (MRD) after cancer treatment. MRD refers to the small number of cancer cells that may remain in the body after therapy, and detecting these cells helps predict relapse risk and adjust treatment strategies. Flow cytometry can detect as few as one leukemic cell in 10,000 normal hematopoietic cells, offering high sensitivity for MRD detection.
Flow cytometry plays a role in managing infectious diseases, such as HIV, by counting specific immune cell populations like CD4 T-cells, which are crucial for immune system health. It also aids in the diagnosis and understanding of autoimmune diseases by identifying abnormal immune cell distributions and activation profiles. For example, it can detect autoantibodies in autoimmune hemolytic anemia with higher sensitivity than some traditional methods.
How Flow Cytometry Works
The process of flow cytometry involves several key steps, starting with sample preparation. Cells from the biological sample must first be suspended in a fluid to create a single-cell suspension, often requiring enzymatic or mechanical disruption for solid tissues. These cells are then stained with fluorescently labeled antibodies, which bind to specific molecules on or inside the cells. Different antibodies can be tagged with different fluorescent dyes, allowing for the simultaneous detection of multiple cellular markers.
Once prepared, the sample is introduced into the flow cytometer’s fluidics system. A sheath fluid surrounds the sample stream, hydrodynamically focusing the cells so they pass one by one through a laser beam. As each cell intercepts the laser, it scatters light, and any fluorescent markers on the cell are excited to emit light.
Detectors within the instrument capture both the scattered light and the emitted fluorescence. Forward scatter (FSC) is collected along the laser’s axis and provides information about cell size, while side scatter (SSC) is collected at a 90-degree angle and indicates the cell’s internal complexity or granularity. The fluorescent signals, each corresponding to a specific labeled marker, are also detected by specialized photomultiplier tubes. This data is then processed by a computer, generating plots and histograms that allow researchers and clinicians to identify, count, and characterize different cell populations within the original sample.