Antigenic composition and classification
The Rh system is based on three main antigens: C, D, and E, which are localized on the erythrocyte membrane. Among them, antigen D has the strongest immunogenic effect — it is its presence or absence that determines a person's Rh status.
In clinical practice, the following classification is used:
- Rhesus-positive blood (Rh+): antigen D is present on the erythrocyte membrane. This is the most common phenotype, found in approximately 85% of people of European descent.
- Rhesus-negative blood (Rh–): antigen D is completely absent on erythrocytes. Such blood is characteristic of approximately 15% of Europeans.
Features of antibody production
A key feature of the Rh system is that congenital antibodies to the D antigen are not detected in human blood plasma.
For immune antibodies to appear in the body, a mandatory condition must be met: the immune system of a Rh-negative person must directly encounter the foreign antigen D. This process of antibody production in response to antigen exposure is called immunization (or sensitization).
Immunization during blood transfusion
One pathway of contact with the antigen is blood transfusion. A problem arises in a situation when blood is transfused from an Rh+ donor to an Rh– recipient.
- First transfusion: the recipient's immune system recognizes the D antigen as foreign and triggers the sensitization process. The production of specific antibodies begins, however, acute destructive reactions generally do not occur at this moment.
- Repeat transfusion: if this same patient is transfused with rhesus-positive blood again in the future, the accumulated antibodies immediately attack the donor's erythrocytes. This leads to their agglutination (clumping) and subsequent hemolysis (destruction), which causes severe complications for the recipient.
Rhesus incompatibility in pregnancy
The second pathway of immunization is pregnancy, in which the mother has rhesus-negative blood (Rh–), and the developing fetus has inherited rhesus-positive (Rh+).
- First pregnancy: normally, the integrity of the placenta is not compromised, so the blood of the mother and fetus does not mix. Since antibodies are initially absent, the pregnancy proceeds without affecting the fetus.
- Childbirth: during labor, some of the child's erythrocytes (Rh+) enter the maternal bloodstream. This provokes the production of maternal anti-D antibodies. These antibodies belong to the immunoglobulin G class (IgG). Their main feature is a low molecular weight, which allows them to cross the placental barrier.
- Subsequent pregnancy (if the fetus is Rh+ again): previously produced maternal antibodies (IgG) freely cross the placenta into the fetal bloodstream. They cause agglutination and hemolysis of the unborn child's erythrocytes. As a result, hemolytic disease develops, which can lead to a severe condition and even fetal death.
Briefly: answers to topical questions
What are the main antigens that form the Rh system?
The Rh system is formed by three antigenic determinants combined into 6 major antigens: Dd, Cc, Ee. The main antigen of the Rh system—antigen D (Rh0(D))—has the strongest immunogenic effect and the greatest practical significance. The symbol "d" is used conventionally to denote the absence of antigen D.
What classes of anti-Rh antibodies are formed during the primary and secondary immune response?
The cited sources directly state for Rh incompatibility: congenital antibodies to antigen D are absent, and after contact with antigen D, an Rh− mother produces anti-D antibodies of the IgG class. These IgGs are capable of crossing the placental barrier and, in a subsequent pregnancy with an Rh+ fetus, cause agglutination and hemolysis of fetal erythrocytes. General sources on the immune response indicate that the primary immune response is characterized by IgM, while IgG predominates in the secondary immune response.
How is specific prophylaxis of Rh incompatibility carried out during pregnancy?
In a pregnant woman with Rh− blood, the husband's/partner's Rh factor is determined to assess the risk of Rh incompatibility. If the partner is Rh−, the risk of Rh incompatibility is absent: further studies of anti-Rh antibody titers and the administration of anti-Rh immunoglobulin are not required. Rh-negative pregnant women undergo anti-Rh antibody screening; if antibodies are detected, the patient is referred to a level III medical organization (perinatal center). Following termination of pregnancy at any gestation age, patients with Rh− blood in the absence of Rh antibodies must intramuscularly receive Rho(D) anti-D immunoglobulin.
Class and exam questions
What is the main difference between the Rh system and the ABO system in terms of antibodies?
In the ABO system, antibodies are congenital. In the Rh system, congenital antibodies to the D antigen are not detected in the plasma — they are formed only after immunization (contact with the antigen).
Why does hemolysis not occur during the first transfusion of Rh+ blood to an Rh– recipient?
Upon the first contact with the D antigen, the recipient's body does not yet have preformed antibodies. Only primary sensitization occurs — the initiation of the immune response and the start of their production.
Why are the antibodies formed after the first delivery dangerous for the next child?
The mother's body produces anti-D antibodies, which belong to the IgG class. They have a low molecular weight, which is why they are able to freely cross the placental barrier into the fetal bloodstream.
Is the fetus protected from Rh incompatibility during the first pregnancy?
Yes, if the integrity of the placenta is not compromised. In this case, the blood of the mother and fetus does not mix, and immunization of the mother does not occur until childbirth.
What else to study on this topic
- Immunogenicity of C and E antigens compared to antigen D
- Mechanisms of erythrocyte agglutination and hemolysis
- Pathogenesis and forms of hemolytic disease of the fetus and newborn
- Structure of the placental barrier and permeability for various immunoglobulin classes
- Methods of preventing Rh sensitization in pregnant women
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Rh system" topic and another 4,128 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Blood" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.