Which immunoglobulin crosses placenta and protects the newborn naturally against bacteria?

RRB Nsg. Superintendent -29 April-2025 (shift-2nd)

Applied Physiology

Easy

Appeared in: RRB Nsg. Superintendent -29 April-2025 (shift-2nd)

Explanation

IgG is the only class of immunoglobulin that is actively transported across the placental barrier from the mother to the fetus.
This process, facilitated by Fc receptors on placental cells, begins around 16 weeks of gestation and increases significantly in the third trimester.
This transfer provides the newborn with 'natural passive immunity,' offering protection against bacterial and viral pathogens to which the mother has been exposed.
This maternal protection is critical for the first 3-6 months of life, bridging the gap until the infant's own immune system can produce sufficient antibodies.

Option A: IgM is a large pentameric molecule, making it physically too large to cross the placental barrier.
Option B: IgA does not cross the placenta. It provides a different form of passive immunity.
Option D: IgD does not cross the placenta and is not involved in providing passive immunity to the newborn.

Visual 1: Diagram - An illustration comparing the structures of the five immunoglobulin classes (IgG, IgM, IgA, IgD, IgE). This visually explains why the large pentameric structure of IgM prevents it from crossing the placenta, unlike the smaller monomeric IgG.
Visual 2: Infographic - A flowchart showing the transfer of maternal IgG across the placenta to the fetus. The infographic can label the Fc receptors on the placenta and show antibodies entering the fetal circulation, clearly demonstrating the mechanism of natural passive 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 in newborns as background academic context rather than a clinical decision trigger.
The principle of IgG transfer is fundamental to neonatal protection. Maternal vaccination (e.g., Tdap) is recommended during pregnancy to boost maternal IgG levels, thereby providing enhanced passive immunity to the newborn against diseases like pertussis (whooping cough).
If a newborn presents with signs of infection, measuring their IgM levels is a key diagnostic step. Elevated IgM confirms a congenital infection, as these antibodies cannot have come from the mother.
Nurses play a vital role in educating new mothers about the benefits of breastfeeding, which includes the transfer of secretory IgA, providing local protection to the infant's gut and reducing the risk of gastrointestinal infections.

How to Approach the Question

First, identify the core concepts in the question: 'immunoglobulin,' 'crosses placenta,' and 'protects the newborn.' This points to the topic of passive immunity.
Recall the five major classes of immunoglobulins and their primary functions and characteristics.
Focus on the key differentiator required by the question: the ability to cross the placenta.
Remember that molecular size is a critical factor. IgG is a monomer (small), while IgM is a pentamer (very large). This makes IgM an unlikely candidate.
Eliminate IgA by recalling that its main role in infant immunity is through breast milk (mucosal immunity), not placental transfer.
Eliminate IgD, as its function is primarily as a B-cell receptor and it exists in very low serum concentrations.

Concept Tested & Keywords

Concept Tested: Placental transfer of immunoglobulins and passive immunity in newborns.
Stem keywords: immunoglobulin, crosses placenta, newborn, protects, bacteria
Lead-in keywords: Which

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