Overview
Glycophorin A (GPA), also known as CD235a, is an integral transmembrane sialoglycoprotein predominantly expressed on the surface of red blood cells (RBCs) and erythroid precursors. It is the most abundant glycoprotein in the erythrocyte membrane and plays a crucial role in maintaining membrane stability, flexibility, and structural integrity of red blood cells.
GPA contains a large extracellular domain rich in sialic acid residues, which contributes to the negative surface charge of RBCs and prevents cell aggregation, thereby promoting smooth blood flow. Beyond its structural role, Glycophorin A is involved in cell adhesion, immune interactions, and host–pathogen interactions, particularly with the malaria parasite Plasmodium falciparum.
Clinically, Glycophorin A is a well-established erythroid lineage marker used in hematopathology, especially in the diagnosis and classification of erythroid disorders and neoplasms.
Pathophysiology and Biological Role
Glycophorin A provides RBC membrane stability and deformability, which is essential for red cells to pass through the microvasculature without damage. Its interaction with other membrane proteins, particularly band 3, supports membrane organization and ion channel regulation.
GPA serves as a receptor for Plasmodium falciparum, facilitating parasite invasion into red blood cells, which makes it a potential malaria vaccine target.
The protein carries the M and N blood group antigens, determined by single amino acid differences. Variations in Glycophorin A form the molecular basis of the MNS blood group system.
Abnormalities in erythrocyte membrane glycoproteins, including GPA, can trigger immune-mediated destruction of RBCs, contributing to autoimmune hemolytic conditions. Altered expression may also be observed as a secondary phenomenon in certain leukemias.
Clinical Indications
Glycophorin A testing has wide diagnostic applications:
- Diagnosis of Acute Erythroid Leukemia (AML M6):
Immunohistochemical detection of GPA confirms erythroid lineage in leukemic blasts. In AML subtypes M0–M5 and M7, GPA is typically negative, whereas it is positive in AML M6. - Assessment of Erythroid Cell Development:
Helps identify normal and abnormal erythroid differentiation in bone marrow and tissue specimens. - Evaluation of Blood Disorders:
Assists in diagnosing and differentiating anemias related to RBC membrane protein defects, such as elliptocytosis, spherocytosis, hereditary pyropoikilocytosis (HPP), stomatocytosis (HSt), and myelodysplastic syndromes (MDS). - Tumor Characterization:
Differentiates erythroid neoplasms from non-erythroid tumors and helps clarify tumor origin in diagnostically ambiguous cases. - Forensic Pathology:
Detects the presence of vital hemorrhage in tissues and helps distinguish ante-mortem from post-mortem lesions, particularly in decomposed samples. - Pre-transfusion Compatibility Testing:
Aids in identifying alloantibodies to prevent incompatible blood transfusions. - Research Applications:
Used to study erythroid lineage, cell surface antigens, and molecular characteristics of RBCs.
Methods of Detection
Multiple laboratory techniques are used to detect Glycophorin A:
- Serological methods: Agglutination tests using GPA-specific antibodies
- Flow cytometry: Fluorescently labeled antibodies detect GPA on RBCs, useful for MN blood typing and assessing GPA mutant frequencies
- Molecular methods: PCR and loop-mediated isothermal amplification (LAMP) for the detection of GPA genetic variants
- Immunohistochemistry (IHC): Confirms the presence of erythroid cells in tissue sections and supports diagnostic interpretation
Sample Collection and Testing Principle
Samples suitable for Glycophorin A testing include:
- Whole blood or bone marrow: 3.0 mL collected in EDTA (lavender-top) or citrate (green-top) tubes and kept at ambient temperature
- Biopsy samples: Preferably submitted in Hank’s fluid
- Liquid biopsy samples: Plasma separated from whole blood with appropriate precautions
- Formalin-fixed paraffin-embedded (FFPE) tissue for IHC
IHC testing uses 10% formalin-fixed paraffin-embedded tissue. The procedure involves binding of a monoclonal primary antibody to the antigen, secondary antibody linkage, chromogenic detection, and microscopic evaluation.
Interpretation and Limitations
Positive staining for CD235a (Glycophorin A) indicates erythroid cells in normal or disease states. Cross-reactivity may be seen with CD235b and combined CD235ab antibodies.
Despite its clinical value, testing has limitations. These include high cost, need for strict clinical correlation, guarded sensitivity and specificity with possible false-negative or confusing results, and dependence on sample quality and timing of testing.
