Currently, there are two major blood group systems commonly used by the medical community to categorize different blood types. One system is the ABO blood group system and the other is the Rh blood group system. The ABO system will not be discussed on this site because it plays no part in Rh incompatibility. For readers who want to learn a little about the ABO system, please check out the American Red Cross website at http://www.redcrossblood.org/learn-about-blood/blood-types .
The Rh blood group system is based solely on the presence or the absence of the Rh D-antigens on the surface of human red blood cells. These Rh-antigens are encoded by genes located on particular regions of human DNA. Every individual receives two copies of each gene, one from each parent; particular variations of the genes, called alleles, determine whether or not an individual will produce the antigen. While between 30 and 50 Rh blood group antigens have been identified (depending on the source), only five Rh antigens are considered significant to Rh incompatibility. These major antigens are encoded by five different alleles—C, c, E, e, and D—on three different genes, located on the human chromosome 1. This website focuses on the D-antigen because the D-antigen is the most common cause of Rh incompatibility. It is quite uncommon for the other four alleles to cause Rh incompatibility. While there is only one allele, D, that produces the D-antigen, a “d” is often used to denote the absence of the D allele. Typically, the gene encoding for D is deleted in an Rh negative individual, but mutations in the gene can also occur and cause the gene to be improperly expressed. Three different genotypes—DD, Dd, and dd—are used to describe which alleles an individual carries. A human is said to be Rh positive if his or her blood possesses the Rh D-antigen (DD or Dd); the individual is Rh negative if the D-antigen is not expressed on the red blood cells’ membranes (dd). Currently, around 85% of the human population is Rh positive while the remaining 15% is Rh negative.