Which is the only immunoglobulin transported through placenta to fetus?

DSSSB - 28 August 2019 (Shift-1)

Obstetrics & Gynaecology

Easy

Appeared in: DSSSB - 28 August 2019 (Shift-1)

Explanation

Immunoglobulin G (IgG) is the only antibody class that crosses the placenta, a process that begins around 16 weeks of gestation and increases significantly during the third trimester.
This transfer is an active process mediated by specific Fc receptors on placental cells, which bind to the Fc portion of the IgG molecule.
Due to its relatively small monomeric structure, IgG can efficiently pass from maternal to fetal circulation, unlike the larger IgM (pentamer) and IgA (dimer) molecules.
This process provides the fetus and newborn with passive immunity, offering protection against infections to which the mother has immunity.
The antibodies transferred reflect the mother's immunological history, including past infections and vaccinations.

Option A: IgD is primarily found on the surface of B-lymphocytes and acts as an antigen receptor. It is present in very low concentrations in the serum and is not involved in placental transport.
Option B: IgM is a large pentameric molecule, making it too big to cross the placental barrier. It is the first antibody produced during a primary immune response.
Option D: IgA is the main immunoglobulin found in mucosal secretions (saliva, tears, intestinal fluid) and is transferred to the newborn through breast milk (colostrum), not the placenta. It provides mucosal immunity.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Placental transfer of immunoglobulins and passive immunity as background academic context rather than a clinical decision trigger.
Understanding that only IgG crosses the placenta is fundamental to neonatal immunology. It explains why newborns have temporary protection against certain diseases.
This knowledge underscores the importance of maternal immunization (e.g., Tdap, influenza vaccine) during pregnancy, as the resulting maternal IgG antibodies are passed to the fetus, protecting the infant in the vulnerable first months of life.
What if the mother has an autoimmune disease mediated by IgG antibodies (e.g., myasthenia gravis, neonatal lupus)? In this case, the pathogenic maternal IgG can also cross the placenta and cause a transient form of the disease in the newborn.

How to Approach the Question

First, identify the core of the question, which is about the transfer of immunoglobulins (antibodies) across the placenta.
Recall the five main classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE.
Access your knowledge about the specific functions and properties of each class. A key distinguishing fact is their ability to cross the placenta.
Remember that IgG is unique in this regard. Its smaller, monomeric structure is a crucial feature that allows for this transport.
Evaluate the other options: IgM is a very large pentamer, making it physically impossible to cross. IgA is associated with mucosal immunity and breast milk. IgD's primary role is as a B-cell receptor.
Based on this process of elimination and direct recall, conclude that IgG is the only immunoglobulin transported through the placenta.

Concept Tested & Keywords

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