Flow Cytometry

Introduction And Core Principles

Flow cytometry is a sophisticated laboratory technique used for immunophenotyping and analyzing the physical and chemical characteristics of cells.
It relies on passing a suspension of cells through a specialized flow cell where they are interrogated by a laser beam.
The technique is heavily utilized in hematology and immunology to establish lineage, evaluate cellular maturity, and detect specific protein deficiencies.
It utilizes specific antibodies conjugated to fluorescent dyes (fluorochromes) that bind to cell surface or intracellular antigens.
These antigens are commonly referred to as Cluster of Differentiation (CD) markers.

Cells are suspended in a saline solution and surrounded by a sheath fluid.
Hydrodynamic focusing forces the cells into a single file line, allowing them to pass through the laser beam one by one.

When the laser strikes a cell, light is scattered in various directions and picked up by detectors via a dichroic mirror and bandpass filters.
Forward Scatter (FSC): Measures the light scattered along the same axis as the laser. This parameter correlates directly with the size of the cell.
Side Scatter (SSC): Measures light scattered at a 90-degree angle. This correlates with the inner complexity and granularity of the cell.

Acute leukemias are characterized by the accumulation of greater than 20% immature blast cells in the bone marrow.
Flow cytometry is crucial for defining the precise lineage of these blasts, distinguishing between acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).
It utilizes immaturity markers like CD34 alongside specific lineage markers.

Leukemia Subtype	Key Flow Cytometry Markers
B-Cell ALL	CD19, CD20, CD22, CD79a, CD10, TdT (Terminal deoxynucleotidyl transferase).
T-Cell ALL	Cytoplasmic CD3, CD4, CD5, CD7, CD8, TdT.
AML	CD13, CD33, CD117, Myeloperoxidase (MPO).

Flow cytometry evaluates minimal residual disease (MRD) following induction chemotherapy in acute lymphoblastic leukemia.
Morphological remission requires less than 5% blasts in the bone marrow.
Flow cytometry provides a much deeper assessment. A blast percentage less than 0.01% indicates an MRD-negative state, which correlates with a better prognosis.

Flow cytometry helps subtype mature B-cell lymphomas.
All B-cell lymphomas are positive for CD20.
Chronic lymphocytic leukemia (CLL) characteristically expresses CD5 and CD23 while being negative for CD10.
Follicular lymphoma expresses CD10 but is negative for CD5.

Multiparameter flow cytometry assesses plasma cell clonality in multiple myeloma.
It determines the burden of neoplastic plasma cells in the bone marrow and evaluates light chain restriction (kappa or lambda).

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired somatic mutation in hematopoietic stem cells leading to the deficiency of Glycosylphosphatidylinositol (GPI) anchored proteins.
Flow cytometry is the investigation of choice for confirming PNH.
It utilizes specific fluorescence-labeled reagents like aerolysin for high sensitivity.
It detects the absence of CD55 (Decay Accelerating Factor) and CD59 (Membrane Inhibitor of Reactive Lysis) on erythrocytes and leukocytes.
A finding of greater than 50% neutrophils lacking CD55/CD59 indicates classic PNH.

Flow cytometry provides absolute and relative counts of distinct lymphocyte populations.
It assesses the normal CD4 to CD8 ratio, which is typically 2:1.

Cell Type	Flow Cytometry Markers
T-Cells	CD3 (total T cells), CD4 (T-helper cells), CD8 (T-cytotoxic cells).
B-Cells	CD19, CD20.
NK Cells	CD16, CD56.
Naive/Memory T-Cells	CD45RA (naive), CD45RO (memory).

The technique evaluates the presence, absence, or functional competence of specific proteins linked to primary immunodeficiencies.

Disorder	Flow Cytometry Findings
Chronic Granulomatous Disease (CGD)	Utilizes the dihydrorhodamine 123 (DHR) test to measure oxidant production.
X-Linked Agammaglobulinemia (XLA)	Absence of B cells (CD19) and lack of Bruton tyrosine kinase (BTK) protein expression.
Leukocyte Adhesion Defect (LAD)	Absent or reduced expression of CD11a, CD11c, and CD18 on lymphocytes and neutrophils.
Wiskott-Aldrich Syndrome (WAS)	Assessment of Wiskott-Aldrich syndrome protein (WASP) expression.
Severe Combined Immunodeficiency (SCID)	Severe T-cell lymphopenia with variable B-cell and NK-cell counts depending on the genetic subtype.
Mendelian Susceptibility to Mycobacterial Disease	Evaluation of the IL-12/23 and Interferon-gamma (IFN-y) pathway receptor expression.
Familial Hemophagocytic Lymphohistiocytosis	Evaluates NK cell cytotoxicity, perforin expression, and degranulation assays.