Introduction

The Rhesus blood group system is of clinical interest because it is involved in the pathogenesis of hemolytic disease of the newborn, hemolytic transfusion reactions, and some autoimmune hemolytic anemias. More than 48 different antigens have been serologically defined, making the Rh system the most polymorphic of all the erythrocyte blood group systems. There are five most frequently typed Rh antigens: C/c, E/e, and the D antigen, which is the most immunogenic, defining an individual as RhD-positive or RhD-negative. Classical hemagglutina-tion used for Rh typing is still a powerful and practical technique, with specificity and sensitivity suitable for clinical applications, but has limitations. Molecular genotyp-ing techniques such as polymerase chain reaction (PCR) can be used to overcome some of these limitations and, in addition, to improve the current level of resolution.

GENETIC BASIS OF THE Rh SYSTEM

The discovery of the Rh blood group system was made just over 60 years ago. A three-gene theory and a singlegene theory had been proposed to explain the inheritance of Rh antigens, but it was in 1991 when Colin et al.[1] demonstrated that the RH locus was composed of two genes and established the genetic basis for RhD-positive and RhD-negative polymorphism, as follows:

The RH locus is composed of two homologous genes denoted RHD and RHCE. Caucasian RhD-positive individuals have either one or two RHD genes per cell, whereas the RhD-negative phenotype is caused by the absence of the entire—or at least part of the—RHD gene (Fig. 1). The RHD gene encodes the RhD protein that expresses the epitopes of the D antigen. The RHCE gene has four prevalent allelic forms: RHCe, Rhce, RhcE, and RHCE, and each allele determines the expression of two antigens in Ce, ce, cE, or CE combination carried by the RhCcEe protein (RHCE is the collective name of the four alleles).

Rh PROTEINS

The Rh proteins consist of 417 amino acids, have 12 transmembrane spans with six extracellular loops, and are involved in the export of ammonium from red blood cells (RBCs).[2,3]

D is the major Rh antigen detected on the surface of RBCs obtained from individuals with presumed genotypes RHD/RHD or RHD/-, commonly referred to as ''RhD-positive.'' It consists of a collection of at least 24 different epitopes expressed on the RhD polypeptide. Whether from RhD-positive or RhD-negative individuals, virtually all normal RBCs bear the antithetical antigens C and/or c (involving the second extracellular loop of the RhCcEe protein), in addition to E and/or e (involving the fourth extracellular loop on the same protein).[2,4,5]

Rh antigens may have a depressed, partial, or ablated expression on the RBC membrane because of allelic variants of the RH genes. A serological weakly reacting form of D was described as weak D because these red cells possess a relatively small number of D antigen sites. Another phenomenon related to D antigen expression is the partial D status associated with both qualitative (lack of one or more D epitopes) and quantitative polymor-phism,[4] which can be identified because of anti-D production and/or lack of reactivity with some monoclonal anti-D. However, molecular analyses have shown that ''weak D'' possesses slightly modified D antigens,[6] suggesting that the terms ''weak D'' and ''partial D'' should be replaced by ''aberrant D'' because each case of altered D expression investigated to date demonstrates a unique D phenotype.[3,5]

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook


Post a comment