The only class of immunoglobulin which is transported across the placenta is?

JODHPUR AIIMS SNO-2018

Obstetrics & Gynaecology

Easy

Appeared in: JODHPUR AIIMS SNO-2018

Explanation

IgG is the only class of immunoglobulin that can be transported across the placenta from the mother to the fetus.
This transfer is an active process mediated by specific neonatal Fc receptors (FcRn) located on placental cells.
It provides the newborn with natural passive immunity, offering protection against infections for the first few months of life until the infant's own immune system matures.
IgG is the most abundant antibody in serum, making up about 75-80% of total immunoglobulins.

Option B: IgA is primarily a secretory antibody found in breast milk, saliva, and tears, providing mucosal immunity. It does not cross the placenta.
Option C: IgD functions mainly as an antigen receptor on the surface of B-lymphocytes and is present in very low concentrations in the serum. It does not cross the placenta.
Option D: IgM is a very large molecule (a pentamer), which physically prevents it from crossing the placental barrier.

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.
The transfer of maternal IgG is crucial for protecting newborns from infections like measles, tetanus, and diphtheria during the first few months of life, before their own immune system fully matures.
This same mechanism can have negative consequences, such as in Rh incompatibility, where maternal IgG antibodies against fetal Rh-positive red blood cells cross the placenta and cause Hemolytic Disease of the Newborn (HDN).
What if? - If a newborn's cord blood tests positive for IgM antibodies, it strongly suggests an active infection acquired in utero (e.g., TORCH infections). This is because the large IgM molecule cannot cross the placenta, so its presence confirms it was produced by the fetus itself.

How to Approach the Question

Identify the key terms in the question: 'only class', 'immunoglobulin', and 'transported across the placenta'.
Recognize this as a factual recall question requiring knowledge of the five main classes of immunoglobulins and their specific properties.
Recall the characteristics of each immunoglobulin: IgG (most abundant, crosses placenta), IgA (secretory), IgM (largest, first responder, does not cross placenta), IgD (B-cell receptor), and IgE (allergies).
Specifically focus on the property of placental transfer. Remember that only one class has this unique ability.
Match the property 'crosses the placenta' with the correct immunoglobulin, which is IgG.
Eliminate the other options based on their known properties: IgM is too large, IgA is secretory, and IgD is a cell surface receptor.

Concept Tested & Keywords

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