IgG is More Important Than IgM for Anti-ABO Titres
When considering anti-ABO titres in the context of transfusion and transplantation, IgG antibodies are clinically more significant than IgM antibodies, as IgG is associated with increased transfusion requirements, delayed engraftment, and hemolytic complications.
Clinical Significance of IgG vs IgM
IgG Antibodies: The Primary Concern
- IgG anti-A/B antibodies are directly associated with increased red blood cell transfusion requirements in bone marrow transplantation with major ABO mismatch, while IgM antibodies show no such correlation 1
- Patients with anti-A/B IgG titers ≥32 required significantly more RBC transfusions compared to those with titers ≤16 (21.29±14.68 vs 6.60±4.55 units), whereas IgM titers had no impact on transfusion needs 1
- High pretransplant IgG titers are associated with increased risk of pure red cell aplasia (PRCA) and delayed red blood cell engraftment following transplantation 1
- IgG antibodies (particularly IgG1/IgG3 subclasses) are associated with total hemolysis and higher titres above 64, making them the critical factor in hemolytic reactions 2
IgM Antibodies: Limited Clinical Impact
- IgM antibodies are not contraindications to transplantation, whether detected by AHG-CDC or flow cytometry crossmatching 3
- IgM anti-A/B titers show no significant association with RBC or platelet transfusion requirements in major ABO-mismatched transplantation 1
- While IgM antibodies can cause positive crossmatches, they are generally considered clinically less relevant than IgG antibodies 3
Practical Management Approach
For ABO-Incompatible Kidney Transplantation
- Successful ABO-incompatible kidney transplantation can be achieved by monitoring and reducing IgM isoagglutinin titers to ≤4 (by immediate spin tube method) during desensitization, regardless of IgG titer 4
- Patients with high pre-transplant IgG isoagglutinin titers (16-256) achieved equally successful outcomes (100% graft survival at 5 years) as those with low IgG titers (≤8) when IgM was adequately controlled 4
- This suggests that while IgG is more pathogenic, adequate control of IgM during the immediate perioperative period can mitigate IgG-mediated complications 4
For Platelet Transfusion Screening
- A critical titre of 64 for IgM anti-A and anti-B can be used as a screening test to prevent transfusion reactions in ABO-minor incompatible platelet transfusions 2
- IgM titres in platelet component plasma correlate well with single-serum samples, making screening practical 2
- IgG subclass testing shows IgG1/IgG3 are associated with hemolysis, while IgG2/IgG4 are not 2
Key Clinical Distinctions
Why IgG Matters More
- IgG antibodies are preformed and persist longer in the circulation, contributing to sustained hemolytic potential 5
- IgG-mediated complement activation leads to more severe and prolonged hemolysis compared to IgM 5
- The antibody response following ABO-incompatible transfusion is predominantly IgG and IgA, with only moderate IgM response 6
Common Pitfalls to Avoid
- Do not assume that low IgM titers alone indicate safety - always assess IgG titers in transplant candidates, as IgG drives long-term complications 1
- Do not rely solely on IgM screening for bone marrow transplantation - IgG titers are the critical predictor of transfusion requirements 1
- In kidney transplantation, while IgM control is the primary desensitization target, recognize that underlying high IgG titers may require more intensive monitoring despite successful outcomes 4
Context-Specific Recommendations
For transplantation risk stratification: Measure both IgG and IgM, but use IgG titers (particularly ≥32) to predict transfusion needs and complications 1
For immediate perioperative management: Focus on reducing IgM titers to ≤4 for safe transplantation, as this approach has demonstrated excellent outcomes even with elevated IgG 4
For platelet transfusion screening: Use IgM titre of 64 as the cut-off for donor screening, as it correlates with hemolysis risk 2