Rh blood group system antigens are encoded by two genes: RHD and RHCE are located on chromosome 1. RHD encodes antigen D, while RHCE encode antigens Cc and Ee. d antigen does not exist; However, by convention, d is used to connote the absence of antigens Rh antigen D function is not known, it appears that Rh antigens play a structural role in the erythrocyte membrane. There are 45 antigens that have been attributed to Rh blood group system, of which the most common and important ones are D, C, c, E and e. After antigens A and B, D antigen shows the highest immunogenicity (20 times higher than other major Rh antigens); after transfusion of D-positive red cells up to 80% of D-negative recipients develop anti-D antibodies. The D antigen is involved in 95% of cases of hemolytic disease of the newborn.

An individual is considered Rh positive if its red cells express the D antigen and the term refers to the absence of negative Rh antigen D. The absence of antigen D is found in withe individuals in a percentage of 15-17% and is less common in other populations. Some erythrocytes expressing the antigen D require prolonged incubation with anti-D reagent for the appearance of agglutination. These erythrocyte are considered antigen D positive and are described as weak D.

Individuals with weak D phenotype by one of these two mechanisms do not form alo-antibodies after exposure to D-positive red cells.

Donors with weak D should be considered Rh (D) positive; it is important that they not be labeled as Rh negative wrong as weak D antigen can induce an immune response in an individual if transfused  to a Rh negative individual.

Rh null phenotype occurs when red cells do not express the Rh antigens.

Anti-Rh appear in aloimmunisation , after incompatible transfusions of Rh system or pregnancy.

Most anti-Rh antibodies are IgG, in particular IgG1 and IgG3 (incomplete agglutinins), although some may be IgM, and usually do not activate complement . IgM antibodies thickeners are usually transient and occur in early immunisation. The most common are anti-D antibodies that may cause hemolytic disease of the newborn and hemolytic severe transfusion reactions. Their frequency has fallen significantly as anti-D prophylaxis in pregnant women with Rh negative and Rh positive fetus. Anti-C, c, E, e is associated with hemolytic disease of the newborn and light hemolytic transfusion reactions.

Erythrocyte Rh printing is done with monoclonal anti-D sera that react in saline and containing both human IgM and IgG polyclonal antibodies, the latter can be used in antiglobulin test for weak D

Recommendations for determining the Rh factor:

• before transfusion;

• antepartum and postpartum immune- hematological monitoring of mother and child;

• in blood donors.

Interpretation of results

Recipients of blood and newborn:

• Rh (D) positive results are obtained if clearly positive (> = 2 +) with both anti-D sera test and self control is negative;

• Rh (D) negative if clearly negative results are obtained with both anti-D test sera, whatever the outcome of self control.

Further testing is mandatory if weak positive results are obtained (<= 2 +) or discordant with test sera.