The Significance of RHD Genotyping and Characteristic Analysis in Chinese RhD Variant Individuals.

Background: RhD is the most important and complex blood group system because of its highly polymorphic and immunogenic nature. RhD variants can induce immune response by allogeneic transfusion, organ transplantation, and fetal immunity. The transfusion strategies are different for RhD variants formed by various alleles. Therefore, extensive investigation of the molecular mechanism underlying RhD variants is critical for preventing immune-related blood transfusion reactions and fetal immunity. Methods: RhD variants were collected from donors and patients in Zhejiang Province, China. The phenotypes were classified using the serologic method. The full coding regions of RHD gene were analyzed using the PCR-SBT method. The multiplex ligation-dependent probe amplification (MLPA) assay was used to analyze the genotype and gene copy number. SWISS-MODLE and PyMOL software were used to analyze 3D structures of RhD caused by the variant alleles. The effect of non-synonymous substitutions was predicted using Polymorphism Phenotyping algorithm (PolyPhen-2), Sorting Intolerant From Tolerant (SIFT), and Protein Variation Effect Analyzer (PROVEAN) software. Results: In the collected RhD variants, 28 distinct RHD variant alleles were identified, including three novel variant alleles. RH-MLPA assay is advantageous for determining the copy number of RHD gene. 3D homology modeling predicted that protein conformation was disrupted and may explain RhD epitope differential expression. A total of 14 non-synonymous mutations were determined to be detrimental to the protein structure. Discussion: We revealed the diversity of RHD alleles present in eastern Chinese RhD variants. The bioinformatics of these variant alleles extended our knowledge of RhD variants, which was crucial for evaluating their impact to guide transfusion support and avoid immune-related blood transfusion reactions.

Missense RHD single nucleotide variants induce weakened D antigen expression by altering splicing and/or protein expression.

BACKGROUND: Although D variant phenotype is known to be due to genetic defects, including rare missense single nucleotide variants (SNVs), within the RHD gene, few studies have addressed the molecular and cellular mechanisms driving this altered expression. We and others showed previously that splicing is commonly disrupted by SNVs in constitutive splice sites and their vicinity. We thus sought to investigate whether rare missense SNVs located in "deep" exonic regions could also impair this mechanism. STUDY DESIGN AND METHODS: Forty-six missense SNVs reported within exons 6 and 7 were first selected from the Human RhesusBase. Their respective effect on splicing was assessed by using an in vitro assay. An RhD-negative cell model was further generated by using the CRISPR-Cas9 approach. RhD-mutated proteins were overexpressed in the newly created model, and cell membrane expression of the D antigen was measured by flow cytometry. RESULTS: Minigene splicing assay showed that 14 of 46 (30.4%) missense SNVs alter splicing. Very interestingly, further investigation of two missense SNVs, which both affect codon 338 and confer a weak D phenotype, showed various mechanisms: c.1012C>G (p.Leu338Val) disrupts splicing only, while c.1013T>C (p.Leu338Pro) alters only the protein structure, in agreement with in silico prediction tools and 3D protein structure visualization. CONCLUSION: Our functional data set suggests that missense SNVs damage quantitatively D antigen expression by, at least, two different mechanisms (splicing alteration and protein destabilization) that may act independently. These data thereby contribute to extend the current knowledge of the molecular mechanisms governing weakened D expression.

HLA-DRB1 molecules and the presentation of anchor peptides from RhD, RhCE, and KEL proteins.

BACKGROUND: Antigens from the Rh and Kell systems are recognized as the most immunogenic in clinical practice. This study evaluated the possible molecular mechanisms involved in the interaction of antigenic peptides with the DRB1 molecules, which help to explain the high frequency of anti-K and association of D + C antibodies in transfusion and incompatible pregnancy. STUDY DESIGN AND METHODS: We included 201 patients with antibodies against antigens from the Rh and Kell systems and compare them with 174,015 controls. HLA-DRB1 genotyping and in silico analysis were performed. The NetMHCIIpan software was used to identify RhD-, RhCE-, and KEL-derived anchor peptides that bind to DRB1 molecules. RESULTS: HLA-DRB1*15 is associated with an increased risk of D, C, E, and K alloimmunization, while the HLA-DRB1*01 and *12 alleles are overrepresented in patients with anti-C and anti-D, respectively. In silico analysis showed that three polymorphic points (60I, 68S, and 103S) common to C and D antigens can be presented by several DRB1 molecules, including DRB1*15:01. The DRB1*09:01 molecule, although not showing statistical significance, was able to interact strongly with almost all five anchor peptides from the sequence containing the polymorphic determinants of E antigen, except 217-WMFWPSVNS-225. CONCLUSION: The DRB1*15 molecule has specific physicochemical characteristics in residues 11P and 13R in the P4 pocket that can favor the response to various antigenic peptides. Anti-K alloimmunization is unrestricted for interaction with specific DRB1 molecules, which suggests that almost all individuals in our population have DRB1 molecules capable of binding to KEL-derived anchor peptides and produce anti-K when stimulated.

Surface-anchored framework for generating RhD-epitope stealth red blood cells.

Rhesus D (RhD) is one of the most important immunogenic antigens on red blood cells (RBCs). However, the supply of RhD-negative blood frequently faces critical shortages in clinical practice, and the positive-to-negative transition of the RhD antigen remains a great challenge. Here, we developed an alternative approach for sheltering the epitopes on RhD-positive RBCs using a surface-anchored framework, which is flexible but can achieve an optimal shield effect with minimal physicochemical influence on the cell. The chemical framework completely obstructed the RhD antigens on the cell surface, and the assessments of both blood transfusion in a mouse model and immunostimulation with human RhD-positive RBCs in a rabbit model confirmed the RhD-epitope stealth characteristics of the engineered RBCs. This work provides an efficient methodology for improving the cell surface for universal blood transfusion and generally indicates the potential of rationally designed cell surface engineering for transfusion and transplantation medicine.

Is the dendritic organ of the striped eel catfish Plotosus lineatus an ammonia excretory organ?

The dendritic organ (DO) is a salt secretory organ in the Plotosidae marine catfishes. The potential role of the DO in ammonia excretion was investigated by examining the effects of salinity [brackishwater (BW 3‰), seawater (SW 34‰) and hypersaline water (HSW 60‰)] acclimation and DO ligation on ammonia excretion and ammonia transporter expression by immunohistochemistry (IHC), immunoblotting (IB) and qPCR. Ammonia flux rates (JAmm) were significantly lower in BW compared to SW and HSW. DO ligation resulted in a significantly lower JAmm in SW but not BW fish. IHC demonstrated apical and basolateral localization of Rhesus-associated glycoprotein (Rhag-like) and Rhbg-like proteins, respectively, in parenchymal cells of the DO acini. In the gills, which are the primary site of ammonia excretion in teleost fishes, IHC showed an apical localization of Rhag-like protein in some Na+/K+-ATPase (NKA) immunoreactive (IR) cells limited to a few interlamellar regions of the filament and, in both apical and basolateral membranes of pillar cells irrespective of treatment group. In gills, the distribution of NKA-IR cells showed no salinity and/or ligation dependency. IB of Rhag and Rhbg-like proteins was found only in the gills and expression levels did not change with salinity but ligation in BW decreased Rhbg-like levels. Although Rhcg was not detected with heterologous antibodies, rhcg1 mRNA expression was detected in both gills and DO. HSW was associated with the lowest expression in DO and ligations in SW and BW were without effect on branchial expression levels. Taken together these results indicate the DO potentially has a physiological role in ammonia excretion under SW conditions.

Molecular and computational analysis of 45 samples with a serologic weak D phenotype detected among 132,479 blood donors in northeast China.

BACKGROUND: RH1 is one of the most clinically important blood group antigens in the field of transfusion and in the prevention of fetal incompatibility. The molecular analysis and characterization of serologic weak D phenotypes is essential to ensuring transfusion safety. METHODS: Blood samples from a northeastern Chinese population were randomly screened for a serologic weak D phenotype. The nucleotide sequences of all 10 exons, adjacent flanking intronic regions, and partial 5' and 3' untranslated regions (UTRs) were detected for RHD genes. Predicted deleterious structural changes in missense mutations of serologicl weak D phenotypes were analyzed using SIFT, PROVEAN and PolyPhen2 software. The protein structure of serologic weak D phenotypes was predicted using Swiss-PdbViewer 4.0.1. RESULTS: A serologic weak D phenotype was found in 45 individuals (0.03%) among 132,479 blood donors. Seventeen distinct RHD mutation alleles were detected, with 11 weak D, four partial D and two DEL alleles. Further analyses resulted in the identification of two novel alleles (RHD weak D 1102A and 399C). The prediction of a three-dimensional structure showed that the protein conformation was disrupted in 16 serologic weak D phenotypes. CONCLUSIONS: Two novel and 15 rare RHD alleles were identified. Weak D type 15, DVI Type 3, and RHD1227A were the most prevalent D variant alleles in a northeastern Chinese population. Although the frequencies of the D variant alleles presented herein were low, their phenotypic and genotypic descriptions add to the repertoire of reported RHD alleles. Bioinformatics analysis on RhD protein can give us more interpretation of missense variants of RHD gene.

Applicability of papain solutions in immunohematology.

OBJECTIVE: To compare the enzyme activity of different presentations of papain solution to validate in-house preparations. METHODS: Two papain solutions were prepared, and the third presentation was a commercial solution. Tests were carried out with samples of red cells typed as weak RhD. RESULTS: In-house prepared papain solutions showed similar enzyme reactivity, and statistically no differences compared to the enzyme activity of the commercial solution. CONCLUSION: Evaluating the cost-benefit ratio, the in-house prepared papain solutions present more economic advantages, and can be incorporated into immunohematological routines as a way to cope with periods of financial crisis and cost-containment policies.

Screening and identification of RhD antigen mimic epitopes from a phage display random peptide library for the serodiagnosis of haemolytic disease of the foetus and newborn.

BACKGROUND: Identification of RhD antigen epitopes is a key component in understanding the pathogenesis of haemolytic disease of the foetus and newborn. Research has indicated that phage display libraries are useful tools for identifying novel mimic epitopes (mimotopes) which may help to determine antigen specificity. MATERIALS AND METHODS: We selected the mimotopes of blood group RhD antigen by affinity panning a phage display library using monoclonal anti-D. After three rounds of biopanning, positive phage clones were identified by enzyme-linked immunosorbent assay (ELISA) and then sent for sequencing and peptides synthesis. Next, competitive ELISA and erythrocyte haemagglutination inhibition tests were carried out to confirm the inhibitory activity of the synthetic peptide. To evaluate the diagnostic performance of the synthetic peptide, a diagnostic ELISA was examined. RESULTS: Fourteen of 35 phage clones that were chosen randomly from the titering plate were considered to be positive. Following DNA sequencing and translation, 11 phage clones were found to represent the same peptide - RMKMLMMLMRRK (P4) - whereas each of the other three clones represented a unique peptide. Through the competitive ELISA and erythrocyte haemagglutination inhibition tests, the peptide (P4) was verified to have the ability to mimic the RhD antigen. The diagnostic ELISA for P4 proved to be sensitive (82.61%) and specific (88.57%). DISCUSSION: This study reveals that the P4 peptide can mimic RhD antigen and paves the way for the development of promising targeted diagnostic and therapeutic platforms for haemolytic disease of the foetus and newborn.

Genomic characterization of the RH locus detects complex and novel structural variation in multi-ethnic cohorts.

PURPOSE: Rh antigens can provoke severe alloimmune reactions, particularly in high-risk transfusion contexts, such as sickle cell disease. Rh antigens are encoded by the paralogs, RHD and RHCE, located in one of the most complex genetic loci. Our goal was to characterize RH genetic variation in multi-ethnic cohorts, with the focus on detecting RH structural variation (SV). METHODS: We customized analytical methods to estimate paralog-specific copy number from next-generation sequencing (NGS) data. We applied these methods to clinically characterized samples, including four World Health Organization (WHO) genotyping references and 1135 Asian and Native American blood donors. Subsequently, we surveyed 1715 African American samples from the Jackson Heart Study. RESULTS: Most samples in each dataset exhibited SV. SV detection enabled prediction of the immunogenic RhD and RhC antigens in concordance (>99%) with serological phenotyping. RhC antigen expression was associated with exon 2 hybrid alleles (RHCE*CE-D(2)-CE). Clinically relevant exon 4-7 hybrid alleles (RHD*D-CE(4-7)-D) and exon 9 hybrid alleles (RHCE*CE-D(9)-CE) were prevalent in African Americans. CONCLUSION: This study shows custom NGS methods can accurately detect RH SV, and that SV is important to inform prediction of relevant RH alleles. Additionally, this study provides the first large NGS survey of RH alleles in African Americans.

Applicability of papain solutions in immunohematology / Aplicabilidade de soluções de papaína em imuno-hematologia

ABSTRACT Objective: To compare the enzyme activity of different presentations of papain solution to validate in-house preparations. Methods: Two papain solutions were prepared, and the third presentation was a commercial solution. Tests were carried out with samples of red cells typed as weak RhD. Results: In-house prepared papain solutions showed similar enzyme reactivity, and statistically no differences compared to the enzyme activity of the commercial solution. Conclusion: Evaluating the cost-benefit ratio, the in-house prepared papain solutions present more economic advantages, and can be incorporated into immunohematological routines as a way to cope with periods of financial crisis and cost-containment policies.

RESUMO Objetivo: Comparar a atividade enzimática de diferentes apresentações de solução de papaína para validação de preparados in-house. Métodos: Foram preparadas duas soluções de papaína, e a terceira apresentação tratou-se de uma solução comercial. Os testes comparativos das reações enzimáticas foram realizados com amostras de hemácias tipadas como RhD fraco. Resultados: As soluções de papaína preparadas in-house apresentaram reatividade enzimática semelhante e estatisticamente sem diferenças em comparação com a atividade enzimática da solução comercial. Conclusão: Avaliando-se a relação entre custo e benefício, as soluções de papaína preparadas in-house são economicamente vantajosas, podendo ser incorporadas às rotinas imuno-hematológicas como forma de enfrentamento em períodos de crise financeira e em políticas de retenção de gastos.

Enhanced opsonisation of Rhesus D-positive human red blood cells by recombinant polymeric immunoglobulin G anti-G antibodies.

BACKGROUND: Anti-RhD antibodies (anti-D) are important in the prophylaxis of haemolytic disease of the foetus and newborn (HDFN) due to RhD incompatibility. Current preparations of anti-D are sourced from hyperimmune human plasma, so its production carries a risk of disease and is dependent on donor availability. Despite the efforts to develop a monoclonal preparation with similar prophylactic properties to the plasma-derived anti-D, no such antibody is yet available. Here we studied the agglutinating, opsonic and haemolytic activities of two recombinant polymeric immunoglobulins (Ig) against the G antigen of the Rh complex. MATERIALS AND METHODS: Recombinant polymeric anti-G IgG1 (IgG1µtp) and IgG3 (IgG3µtp) were produced in vitro, purified by protein G-affinity chromatography, and analysed by gel electrophoresis. Their agglutinating, opsonic and haemolytic activities were evaluated using haemagglutination, erythrophagocytosis, and complement activation assays. RESULTS: The recombinant IgG1µtp and IgG3µtp anti-G antibodies ranged from 150,000 to 1,000,000 Da in molecular weight, indicating the formation of polymeric IgG. No complement activation or haemolytic activity was detected upon incubation of RhD-positive red-blood cells with the polymeric anti-G IgG. Both polymers were better opsonins than a prophylactic preparation of plasma-derived anti-D. DISCUSSION: The enhanced opsonic properties of the polymeric anti-G IgG1µtp and IgG3µtp could allow them to mediate the clearance of RhD-positive red blood cells from circulation more efficiently than natural or other synthetic prophylactic anti-D options. Their inability to induce complement-mediated haemolysis would be prophylactically convenient and is comparable in vitro to that of the available plasma-derived polyclonal anti-D preparations. The described properties suggest that polymeric antibodies like these (but with anti-D specificity) may be testable candidates for prophylaxis of HDFN caused by anti-D.

Combination peptide immunotherapy suppresses antibody and helper T-cell responses to the RhD protein in HLA-transgenic mice.

The offspring from pregnancies of women who have developed anti-D blood group antibodies are at risk of hemolytic disease of the newborn. We have previously mapped four peptides containing immunodominant T-helper cell epitopes from the RhD protein and the purpose of the work was to develop these into a product for suppression of established anti-D responses. A panel of each of the four immunodominant RhD peptides was synthesized with modifications to improve manufacturability and solubility, and screened for retention of recognition by human T-helper cells. A selected version of each sequence was combined in a mixture (RhDPmix), which was tested for suppressive ability in a humanized murine model of established immune responses to RhD protein. After HLA-DR15 transgenic mice had been immunized with RhD protein, a single dose of RhDPmix, given either intranasally (P=0.008, Mann-Whitney rank sum test) or subcutaneously (P=0.043), rapidly and significantly suppressed the ongoing antibody response. This was accompanied by reduced T-helper cell responsiveness, although this change was less marked for subcutaneous RhDPmix delivery, and by the recruitment of cells with a regulatory T-cell phenotype. The results support human trials of RhDPmix peptide immunotherapy in women with established antibody responses to the RhD blood group.

Mice expressing RHAG and RHD human blood group genes.

Anti-RhD prophylaxis of haemolytic disease of the fetus and newborn (HDFN) is highly effective, but as the suppressive mechanism remains uncertain, a mouse model would be of interest. Here we have generated transgenic mice expressing human RhAG and RhD erythrocyte membrane proteins in the presence and, for human RhAG, in the absence, of mouse Rhag. Human RhAG associates with mouse Rh but not mouse Rhag on red blood cells. In Rhag knockout mice transgenic for human RHAG, the mouse Rh protein is "rescued" (re-expressed), and co-immunoprecipitates with human RhAG, indicating the presence of hetero-complexes which associate mouse and human proteins. RhD antigen was expressed from a human RHD gene on a BAC or from RHD cDNA under control of ß-globin regulatory elements. RhD was never observed alone, strongly indicative that its expression absolutely depends on the presence of transgenic human RhAG. This first expression of RhD in mice is an important step in the creation of a mouse model of RhD allo-immunisation and HDFN, in conjunction with the Rh-Rhag knockout mice we have developed previously.

SNPs altering ammonium transport activity of human Rhesus factors characterized by a yeast-based functional assay.

Proteins of the conserved Mep-Amt-Rh family, including mammalian Rhesus factors, mediate transmembrane ammonium transport. Ammonium is an important nitrogen source for the biosynthesis of amino acids but is also a metabolic waste product. Its disposal in urine plays a critical role in the regulation of the acid/base homeostasis, especially with an acid diet, a trait of Western countries. Ammonium accumulation above a certain concentration is however pathologic, the cytotoxicity causing fatal cerebral paralysis in acute cases. Alteration in ammonium transport via human Rh proteins could have clinical outcomes. We used a yeast-based expression assay to characterize human Rh variants resulting from non synonymous single nucleotide polymorphisms (nsSNPs) with known or unknown clinical phenotypes and assessed their ammonium transport efficiency, protein level, localization and potential trans-dominant impact. The HsRhAG variants (I61R, F65S) associated to overhydrated hereditary stomatocytosis (OHSt), a disease affecting erythrocytes, proved affected in intrinsic bidirectional ammonium transport. Moreover, this study reveals that the R202C variant of HsRhCG, the orthologue of mouse MmRhcg required for optimal urinary ammonium excretion and blood pH control, shows an impaired inherent ammonium transport activity. Urinary ammonium excretion was RHcg gene-dose dependent in mouse, highlighting MmRhcg as a limiting factor. HsRhCG(R202C) may confer susceptibility to disorders leading to metabolic acidosis for instance. Finally, the analogous R211C mutation in the yeast ScMep2 homologue also impaired intrinsic activity consistent with a conserved functional role of the preserved arginine residue. The yeast expression assay used here constitutes an inexpensive, fast and easy tool to screen nsSNPs reported by high throughput sequencing or individual cases for functional alterations in Rh factors revealing potential causal variants.

Characterization of novel RHD alleles: relationship between phenotype, genotype, and trimeric architecture.

BACKGROUND: RH1 is one of the most clinically important blood group antigens in the field of transfusion and prevention of fetomaternal incompatibilities. New variant RHD alleles are regularly identified and their characterization is essential to ensuring patient safety. STUDY DESIGN AND METHODS: Blood samples with uncertain RhD phenotypes not resolved by our first-line SNaPshot assay were sequenced for all 10 RHD exons. RHD zygosity was investigated. Flow cytometry was performed to determine RhD antigen density and epitope pattern. RESULTS: Seven novel RHD alleles were identified. Six, that is, RHD(T55P), RHD(A85G), RHD(G132R), RHD(G132E), RHD(D403V), and DAR(T203A), resulted from nucleotide polymorphisms. The seventh, that is, RHD(S182WfsX46), resulted from a 4-bp deletion that led to a reading frame shift and the appearance of a premature stop codon. Study of RhD expression of the first five alleles at hemizygous state showed greatly reduced antigen densities ranging from 50 to 618 antigens per red blood cell (RBC). DAR(T203A) was classified as a partial D antigen with a weakened reactivity profile similar to that of DAR. As expected, no D antigen was detected on RBCs carrying the RHD(S182WfsX46) allele. In parallel, RhD expression of RHD(G336R)/weak D type 58, RHD(F410V), and suspected RHD(1-9)-CE was determined to be less than or equal to 50 antigens per RBC. RhAG/RhD(2) trimer model supports the observed phenotypes. CONCLUSION: Although the frequency of the new RHD alleles presented herein is low, their phenotypic and genotypic description adds to the repertoire of reported RHD alleles. These data can be useful for optimization of molecular screening tools.

Direct current insulator-based dielectrophoretic characterization of erythrocytes: ABO-Rh human blood typing.

A microfluidic platform developed for quantifying the dependence of erythrocyte (red blood cell, RBC) responses by ABO-Rh blood type via direct current insulator dielectrophoresis (DC-iDEP) is presented. The PDMS DC-iDEP device utilized a 400 x 170 µm² rectangular insulating obstacle embedded in a 1.46-cm long, 200-µm wide inlet channel to create spatial non-uniformities in direct current (DC) electric field density realized by separation into four outlet channels. The DC-iDEP flow behaviors were investigated for all eight blood types (A+, A-, B+, B-, AB+, AB-, O+, O-) in the human ABO-Rh blood typing system. Three independent donors of each blood type, same donor reproducibility, different conductivity buffers (0.52-9.1 mS/cm), and DC electric fields (17.1-68.5 V/cm) were tested to investigate separation dependencies. The data analysis was conducted from image intensity profiles across inlet and outlet channels in the device. Individual channel fractions suggest that the dielectrophoretic force experienced by the cells is dependent on erythrocyte antigen expression. Two different statistical analysis methods were conducted to determine how distinguishable a single blood type was from the others. Results indicate that channel fraction distributions differ by ABO-Rh blood types suggesting that antigens present on the erythrocyte membrane polarize differently in DC-iDEP fields. Under optimized conductivity and field conditions, certain blind blood samples could be sorted with low misclassification rates.

Explorations of ABO-Rh antigen expressions on erythrocyte dielectrophoresis: changes in cross-over frequency.

A quadrupole dielectrophoretic microdevice was utilized to examine the ABO-Rh dependencies on erythrocyte polarizations. This important step toward medical microdevice technology would transform key clinical blood tests from the laboratory into the field. Previous work in dielectrophoretic microdevices demonstrated that the large number of ABO antigens on erythrocyte membranes impacts their dielectrophoretic signature at 1 MHz. This work explores the dielectrophoretic behavior of native human erythrocytes categorized by their ABO-Rh blood types and directly compares these responses to the same erythrocyte sample modified to remove the A and B antigens. A ß(1-3)-galactosidase enzyme was utilized to cleave the ABO polysaccharide backbone at the galactosidase bonds. The enzymatic reaction was optimized by comparing agglutination of the native and modified blood cells in addition to UV-Vis and HPLC analysis of the reaction effluent for saccharide residues. Next, the dielectrophoretic behaviors of the native and modified erythrocytes were visually verified in a quadrupole electrode microdevice over a frequency range from 100 kHz to 80 MHz. The lower cross-over frequency (COF), which transitions from negative to positive dielectrophoresis, for ABO blood types tested (A+, A-, B+, B-, AB+, O+ and O-) differed over the range from 17 to 47 MHz. The COFs of the corresponding enzyme-modified erythrocytes were also determined and the range narrowed to 29-41 MHz. A second COF in the 70-80 MHz range was observed and was reduced in the presence of the transmembrane Rhesus factor. These results suggest that antigen expression on erythrocyte membrane surfaces influence cell polarizations in nonuniform AC fields.

Molecular genetics and clinical applications for RH.

Rhesus is the clinically most important protein-based blood group system. It represents the largest number of antigens and the most complex genetics of the 30 known blood group systems. The RHD and RHCE genes are strongly homologous. Some genetic complexity is explained by their close chromosomal proximity and unusual orientation, with their tail ends facing each other. The antigens are expressed by the RhD and the RhCE proteins. Rhesus exemplifies the correlation of genotype and phenotype, facilitating the understanding of general genetic mechanisms. For clinical purposes, genetic diagnostics of Rhesus antigens will improve the cost-effective development of transfusion medicine.