Determination of IgG1 and IgG3 subclasses of red blood cell antibodies: An important tool for predicting harmfulness

Immune-mediated haemolysis caused by red blood cell (RBC) auto or alloantibodies depends on several factors, including antibody subclass. Immunoglobulin (Ig) IgG1 and IgG3 are more efficient at triggering phagocytosis and complement activation. This study evaluated whether IgG subclass determination can help predict the clinical relevance of RBC antibodies in different immunohaematological contexts.

Blood donors and patients with IgG-positive direct antiglobulin tests (DATs) were included. IgG subclasses were determined using monospecific gel cards for IgG1/IgG3. The monocyte monolayer assay (MMA) assessed in vitro biological relevance. Antibody specificity was established by standard immunohaematological techniques. Statistical comparisons were performed using Chi-square, Fisher's exact and Mann-Whitney U tests.

Among patients with IgG autoantibodies (n = 29), 51.7% had IgG1 or IgG3, versus 3.7% of donors (n = 27; p < 0.001). The presence of IgG1/IgG3 autoantibodies showed 93% positive predictive value (PPV) and 96.3% specificity for distinguishing patients from donors. IgG1/IgG3 autoantibodies were more frequently associated with positive MMA results (83.3% vs. 33.3%). Among RBC alloantibodies (n = 17), 64% were IgG1/IgG3, correlating with MMA positivity (sensitivity 78%; PPV 77%). Antibodies traditionally considered benign were often IgG1/IgG3 and MMA-positive.

IgG subclass determination provides diagnostic value beyond IgG quantification alone. In DAT-positive donors, not detecting IgG1/IgG3 is compatible with a low haemolysis risk and often obviates follow-up; when IgG1/IgG3 are detected, selective evaluation may be appropriate. For alloantibodies, subclass identification may help predict clinical relevance, especially in urgent transfusion settings when MMA is unavailable, supporting transfusion safety decisions and efficient resource use.