The immunoglobulin that can cross the placenta and transfer the mother's immunity to the developing foetus is?

DSSSB -28 August 2019 (Shift-2)

Applied Microbiology & Infection control

Easy

Appeared in: DSSSB -28 August 2019 (Shift-2)

Explanation

Immunoglobulin G (IgG) is the only class of antibody that is actively transported across the placenta from the mother to the fetus.
This transfer is facilitated by the neonatal Fc receptor (FcRn) on placental cells and begins around 16 weeks, increasing significantly during the third trimester.
This process provides the newborn with passive immunity, offering protection against infections for the first several months of life while their own immune system matures.
The relatively small, monomeric structure of IgG allows it to pass through the placental barrier, unlike other larger immunoglobulins.

Option B: Immunoglobulin M (IgM) is a large pentameric molecule, which is too big to cross the placental barrier.
Option C: Immunoglobulin D (IgD) is primarily found on the surface of B cells and is present in very low concentrations in the blood. It does not have a role in placental transfer.
Option D: Immunoglobulin A (IgA) does not cross the placenta.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Maternal-Fetal Immunity and Placental Transfer of Immunoglobulins as background academic context rather than a clinical decision trigger.
Understanding passive immunity is vital for neonatal nursing care. Maternal IgG levels in an infant naturally decline over the first 3-6 months, which explains the timing of the primary infant vaccination schedule, designed to stimulate the infant's own active immunity as maternal protection wanes.
This principle is the basis for recommending maternal vaccinations during pregnancy, such as the Tdap (tetanus, diphtheria, pertussis) vaccine. This boosts the mother's IgG levels, ensuring maximum transfer of protective antibodies to the newborn against whooping cough.
What if? If a newborn's blood test reveals elevated levels of IgM, a nurse should anticipate and prepare for further screening for congenital infections like cytomegalovirus, toxoplasmosis, or rubella. This is because elevated IgM indicates an active infection in the fetus, not passive immunity from the mother.

How to Approach the Question

First, identify the key concept in the question: which antibody crosses the placenta to provide immunity to the fetus.
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE (mnemonic: GAMED).
Associate each immunoglobulin with its primary function and location. IgG is the most abundant in blood and is known for crossing the placenta.
Remember that IgM is a large molecule ('M' for 'Mega' or 'Macro') and cannot cross the placenta.
Remember that IgA is the 'secretory' antibody, found in milk, tears, and saliva, but it does not cross the placenta.
Based on these key facts, conclude that IgG is the only immunoglobulin that provides passive immunity to the fetus via placental transfer.

Concept Tested & Keywords

Concept Tested: Maternal-Fetal Immunity and Placental Transfer of Immunoglobulins
Stem keywords: immunoglobulin, cross the placenta, mother's immunity, fetus
Lead-in keywords: is

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E6 Obstetrics Williams p. 29-49

E6 Parks TextBook of Preventive & Social Medicine part 1 — Subpart A (pp 26-201 of 604) p. 101-103