What is Rhesus (Rh) typing?

Updated: Aug 01, 2018
  • Author: Victoria K Gonsorcik, DO; Chief Editor: Jun Teruya, MD, DSc, FCAP  more...
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Answer

After the ABO system, the Rh (Rhesus) blood group system is regarded as the second most important blood group system, as some of the severe hemolytic transfusion reactions and most hemolytic disease of the fetus and newborn (HDFN) cases are associated with antibodies to the Rh group antigens.

The Rh system consists of over 50 red cell antigens. There are five main Rh red cell antigens—D, C, c, E, and e—that involve most clinically significant transfusion complications. These five are the focus of this article.

Two separate genes for the Rh system are found on chromosome 1. One gene, RHD, encodes for the D antigen. Individuals with the D antigen present on their red blood cells are labeled as "Rh (D)–positive." Those who do not have the D antigen are labeled as "Rh (D)–negative."

The frequency of "Rh-negative" individuals varies among different ethnic groups, largely because of the different molecular mechanisms that cause the absent expression of the D antigen. For example, 15% of white individuals who are Rh-negative have this finding mainly caused by the absence of the RHD gene. Approximately 6% of blacks and less than 1% of Asians are Rh-negative. Mechanisms of Rh-negativity in these ethnic groups may include gene deletion, gene rearrangement, gene duplication, and gene mutation. It is important to note that a precise typing of the RH genotype can be achieved with the advancement of molecular testing techniques. [1, 2]

A second gene, RHCE, encodes for a combination of CE or ce antigens together. RHD and RHCE are highly homologous to each other. In essence, RHCE is the original gene, and RHD is a duplication of RHCE. The RHD and RHCE genes encode proteins such that each crosses the red cell membrane 12 times, with 6 protein loops on the exterior of the cell membrane.


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