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Chapter 20 Rh Alloimmunization
Jennifer Deirmengian, MD
❍ What is Rh factor?
Rh refers to the immunogenic D antigen, an erythrocyte surface antigen of the rhesus (Rh) blood group system.
❍ What is alloimmunization?
The development of antibodies in response to alloantigens, antigens present in other patients but not the patient
who develops antibodies. This circumstance commonly occurs when a gravid patient develops antibodies to fetal
red blood cell antigens.
❍ What type of maternal antibody can cross the placenta?
Immunoglobulin G (IgG).
❍ What are the two systems of nomenclature for the Rh blood group system?
Fisher-Race and Weiner.
❍ What is the Fisher-Race nomenclature with respect to Rh alloimmunization?
The nomenclature assumes that there are three genetic loci with two major alleles each. The antigens produced by
these alleles have a letter—C, c, D, E, and e (no “d” has been identified, but it is used to indicate an absence of an
allele product). The most common genotypes are Cde/cde and CDe/Cde. The majority of Rh isoimmunization
is caused by D antigen. Thus, Rh-positive has come to represent the presence of the D antigen and Rh-negative
indicates the absence of D antigen on erythrocytes.
❍ On what chromosome is the genetic locus of the Rh antigen?
Chromosome 1 (the short arm).
❍ What percentage of individuals are Rh-positive?
Rh positivity varies by race.
Caucasians: 85%.
African Americans: 92% to 95%.
Asians and Native Americans: 98% to 99%.