Rhesus (Rh) in Blood Group Types
The Rhesus (Rh) system was first demonstrated by testing human blood with rabbit antisera against red cells of Rhesus monkeys, classifying individuals as Rh-positive or Rh-negative. Despite being widely used clinically, the underlying biochemical genetics remains somewhat confusing.
Genetics of the Rh System
- Rh genotype is determined by three pairs of closely linked allelic genes on chromosome 9: D/d, C/c, E/e (Fisher-Race theory).
- The d gene is amorphous and has no antigenic expression, leaving five effective antigens: D, C, c, E, e.
- Weiner proposed a single locus with allelic series: Rho (D), rh (C), rh (E), hr (C), hr (e).
- Rosenfield’s updated system refers to these antigens as Rh1, Rh2, Rh3, Rh4, Rh5.
- Less common antigens include Cw, Du, Es.
- Foetus inherits one haplotype from each parent, e.g., Cde, cde.
- 12 sets of combinations → 78 possible genotypes.
- Most frequent genotypes:
- Cde/cde (33%)
- Cde/cDe (18%)
- Cde/cDE (12%)
- cDE/cde (11%)
- cde/cde (15%)
- cdE/cde (1%)
- Cde/cde (1%)
- For clinical purposes, knowing whether an individual is Rh-positive or Rh-negative against anti-D sera is usually sufficient.
Incidence of Rh-negative Blood
- Mongoloids: 0%
- Chinese/Japanese: 1–2%
- Indians: 5%
- Africans: 5–8%
- Caucasians: 15–17%
- Basques: 30–35%
Antigenic Expression
- Expression depends on interaction between RBC membrane proteins and phospholipids, forming epitopes corresponding to C/c, D/d, E/e.
- D antigen is highly immunogenic; C/c and E/e are weak and often impractical to match.
- D antigen has no natural antibody, while C and E have weak natural antibodies.
- Rh null is a rare type lacking all known Rh antigens.