Immunoglobulin that can cross the placental barrier and reach the fetal circulation is

Appeared in: BHU NO-2019

Explanation

IgG is the only class of immunoglobulin that is actively transported across the placental barrier.
This transfer is facilitated by specific neonatal Fc receptors (FcRn) on placental cells, starting around 16 weeks and increasing throughout gestation.
It provides essential passive immunity to the fetus and newborn, protecting them from infections during the first few months of life while their own immune system matures.

Option A: IgA does not cross the placenta. It is primarily found in mucosal secretions and is transferred to the infant via breast milk to provide local immunity.
Option C: IgD does not cross the placenta. It exists in very low concentrations in the serum and its primary role is as a surface receptor on B lymphocytes.
Option D: IgM is a large pentameric molecule, which is physically too big to pass through 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.
Understanding that only IgG crosses the placenta is fundamental to neonatal care. The passive immunity from maternal IgG protects the newborn while their own immune system is developing.
The absence of IgM transfer is clinically significant. If IgM is detected in a newborn's blood, it indicates an active infection acquired in utero (e.g., rubella, toxoplasmosis, cytomegalovirus), as the fetus must have produced it.
What if? If a mother has an autoimmune disease caused by pathogenic IgG antibodies (e.g., myasthenia gravis, neonatal lupus), these harmful antibodies can also cross the placenta and cause a temporary form of the disease in the newborn.

How to Approach the Question

Identify the core concept of the question: It asks about the specific immunoglobulin that can cross the placenta to the fetus.
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE.
Systematically review the primary function and key characteristics of each class, focusing on the property of placental transfer.
Remember the mnemonic 'G for Gestation' to associate IgG with its unique ability to cross the placenta during pregnancy.
Eliminate the other options based on their distinct properties: IgA is for secretions, IgM is too large and indicates acute infection, and IgD is a B-cell receptor.
Conclude that IgG is the only immunoglobulin that provides passive immunity to the fetus by crossing the placental barrier.

Concept Tested & Keywords

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E6 Obstetrics Williams p. 29-50

E6 Microbiology Essentials Sastry 4e Part 1 p. 149-151

Practise the full BHU NO-2019

Attempt every question from this paper in a timed mock, then review the full solution for each one.