Which antibody can transfer from mother to fetus?

SGPGI Nursing Officer-2022

Obstetrics & Gynecology

Easy

Appeared in: SGPGI Nursing Officer-2022

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 mediated by specific neonatal Fc receptors (FcRn) on placental cells and begins around 16 weeks, peaking in the third trimester.
This process provides the newborn with crucial passive immunity, protecting it from infections to which the mother is immune for the first several months of life.
The newborn's IgG level at birth is approximately equal to the mother's, reflecting the mother's immunological history.

Option B: IgM is a large pentameric molecule, making it too big to cross the placental barrier.
Option C: IgA does not cross the placenta. Its primary route of transfer to the infant is through breast milk, particularly the colostrum.
Option D: IgE is primarily associated with allergic (type I hypersensitivity) reactions and defense against parasites. It 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 transfer of immunoglobulins and passive immunity as background academic context rather than a clinical decision trigger.
The principle of IgG transfer is the foundation for maternal immunization strategies. Administering vaccines like Tdap and inactivated influenza during pregnancy boosts maternal IgG levels, which are then passed to the fetus, providing vital protection in early infancy before the baby can be vaccinated.
This mechanism is also responsible for hemolytic disease of the fetus and newborn (HDFN). In an Rh-negative mother carrying an Rh-positive fetus, maternal anti-D IgG antibodies can cross the placenta and destroy fetal red blood cells.
What if? If a baby is born very prematurely (e.g., at 28 weeks), the amount of transferred maternal IgG is significantly lower because the bulk of the transfer happens late in pregnancy. This places the preterm infant at a much higher risk for serious infections compared to a full-term infant.

How to Approach the Question

This is a factual recall question testing fundamental knowledge of immunology related to pregnancy.
First, identify the core concept of the question: the transfer of antibodies from mother to fetus across the placenta.
Recall the different classes of immunoglobulins (antibodies): IgG, IgA, IgM, IgE, and IgD.
Focus on the unique properties of each class, especially their size and ability to cross biological barriers.
Remember that IgG is the most abundant immunoglobulin in the blood and is structurally the smallest, which facilitates its transport.
Eliminate the other options based on their characteristics: IgM is a large pentamer and cannot cross; IgA is primarily transferred via secretions like breast milk; and IgE is involved in allergies.

Concept Tested & Keywords

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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