Understanding the Immunobiology of Weak D and D Variants

Introduction

• Speaker: Dr. M Chri, Senior Consultant, New Delhi
• Event: Transconnect April 2024
• Topic: Immunobiology of Weak D and D Variants
• Areas Covered: RH system, genetics, biochemistry, RH antigens, RH genotyping, RH deficiency syndromes

Importance of RH System

• Second most important blood group system after AO in transfusion medicine
• Immunogenic, complex, and polymorphic (many forms)
• Characterized 55 antigens with 5 principal antigens: D, C, E, c, e
• No small D antigen

D Antigen

• D Positive/Negative: Refers to the presence/absence of the RH antigen D
• Importance: Avoid transfusing RH positive blood to RH negative individuals, crucial during pregnancy to prevent sensitization of RH negative mothers

Historical Landmarks

• 1939: Levine and Sten - HDR and common factor in fetus and father
• 1940: Landsteiner and Weiner - Differentiation of RS positive/negative using guinea pig antiserum

Genetics of RH Genes

• Location: Chromosome 1
  • RHD gene: Codes for rhd protein
  • RHCE gene: Codes for RH C, c, E, e proteins
• Codominance: Both genes are expressed
• Mutations: Over 250 alleles in RHD gene and 50 alleles in RHCE gene. These are rare and do not usually change serology.
• Structure: Both genes have 10 exons each
• RHAG Gene: Located on chromosome 6, regulates RH antigen expression, and is glycosylated

RH Membrane Complex

• Components: RhD, RhCE, RHAG, protein 4.2, ankrin, spectrin, Band 3, CD47
• Importance: Maintains RBC membrane integrity; defects can lead to RBC shape abnormalities (stomatocytes, elliptocytes, spherocytes)

When Is an Individual RH Positive?

• Single/Multiple Copies: Presence of at least one RHD gene means RH positive
• D Negative Phenotypes: Deletion (common in Europeans), mutation (common in Asians), or pseudo-gene (common in Africans)
• Immunogenicity: Even small amounts (0.1 mL) of RH positive RBCs can trigger antibody production in RH negative individuals

Weak D Variation

• Definition: Reduced expression strength of D antigen, historical term DU typing now referred to as weak D
• Mechanisms:
  • C in trans to RHD: Position effect reducing D antigen expression
  • Genetic Weak D: Reduced D antigen density due to mutations
  • Partial D: Missing/altered epitopes, qualitative defect, can form antibodies

Molecular Basis and Typing

• Molecular Typing: Using PCR and single nucleotide polymorphism (SNP) to identify VD types
• Significance of VD Types 1, 2, 3: Behave like RHD positive, not at risk of forming anti-D, not candidates for RhIG
• Other Types: May behave differently, require careful typing and RhIG administration in pregnant women

Partial D Antigen

• Hybrid Genes: Result from gene conversion involving RHD and RHCE genes
• Antibody Production: Can form antibodies to the missing part of D antigen
• Clinical Use: Different management in donors (D positive) and recipients (D negative)

Important Notes on RH Deficiency

• RH Null Syndrome: Lack of RH antigens
  • Complications: Hemolytic anemia, skeletal RBC membrane defects
• RH Mod Condition: Altered expression due to mutations in RHAG gene

Conclusion

• Impact: Proper understanding and molecular genotyping are crucial for managing hemolytic disease of the newborn and transfusion reactions effectively.
• Thank you for attending the lecture.