Which statement accurately describes the mechanism of Rh isoimmunization?

GMCH Nursing Officer-2025

Obstetrics & Gynaecology

Hard

Appeared in: GMCH Nursing Officer-2025

Explanation

This statement correctly identifies the key players: an Rh-negative mother and an Rh-positive fetus.
It accurately describes the sensitization process, where the mother's immune system is exposed to fetal Rh-positive red blood cells.
Crucially, it specifies that IgG is the antibody class responsible for the disease, as IgG is small enough to cross the placenta and attack fetal red blood cells.
This process leads to hemolytic disease of the fetus and newborn (HDFN) in subsequent pregnancies with an Rh-positive fetus.

Option A: IgM antibodies are large pentameric molecules and are physically unable to cross the placental barrier to reach the fetal circulation.
Option B: This statement reverses the direction of the immune attack. The fetus's immune system does not attack maternal cells in this condition. It is the mother's antibodies that attack the fetus's cells.
Option C: For isoimmunization to occur, there must be an antigen mismatch where the mother lacks an antigen that the fetus possesses. An Rh-positive mother already has the Rh antigen and will not mount an immune response against an Rh-negative fetus, which lacks the antigen.

Visual 1: Diagram: A two-part diagram showing (1) the first pregnancy of an Rh-negative mother with an Rh-positive fetus, leading to sensitization at birth, and (2) the second pregnancy where maternal IgG antibodies cross the placenta and cause hemolysis in the new fetus.
Visual 2: Flowchart: A flowchart illustrating the sequence of events in Rh isoimmunization, from initial exposure to fetal RBCs, to IgM production, class switching to IgG, and subsequent hemolysis in the next pregnancy.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pathophysiology of Rh isoimmunization as background academic context rather than a clinical decision trigger.
The primary nursing intervention to prevent Rh isoimmunization is the administration of Rho(D) immune globulin (e.g., RhoGAM) to Rh-negative mothers.
RhoGAM is given around 28 weeks of gestation and within 72 hours after delivery of an Rh-positive infant, or after any potential sensitizing event like a miscarriage or amniocentesis.
Nurses are responsible for identifying at-risk mothers through blood typing, administering RhoGAM correctly, and educating the patient on its importance for future pregnancies.

How to Approach the Question

First, identify the core concept: the mechanism of Rh isoimmunization. This is a pathophysiology question.
Break down each option into its key components: (1) Mother's Rh status, (2) Fetus's Rh status, (3) Type of antibody (IgM or IgG), and (4) Action of the antibody (e.g., crossing the placenta).
Recall the fundamental principles: The mother must be Rh-negative and the fetus Rh-positive for sensitization to occur.
Remember the difference between IgM and IgG antibodies. IgM is the first responder but is large and cannot cross the placenta. IgG is part of the secondary, memory response and is small enough to cross the placenta.
Evaluate each option against these principles. The correct option will have all components aligned with the known pathophysiology of the disease.

Concept Tested & Keywords

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