The immunoglobulin that crosses placenta is?

AIIMS Bhuvneshwar NO- 2018

Obstetrics & Gynaecology

Easy

Appeared in: AIIMS Bhuvneshwar NO- 2018

Explanation

Immunoglobulin G (IgG) is the only class of antibody that is actively transported across the placenta.
This transfer is mediated by the neonatal Fc receptor (FcRn) on placental cells, a process that begins around 16 weeks and peaks late in the third trimester.
The primary purpose of this transfer is to provide the fetus and newborn with passive immunity against infections that the mother has been exposed to.
Due to its relatively smaller size (monomer structure, ~150,000 Da), it is capable of being transported, unlike larger immunoglobulins.

Option B: IgE is primarily involved in allergic (hypersensitivity) reactions and defense against parasitic worms. It does not cross the placenta.
Option C: IgM is a large pentamer molecule (five units joined together, ~900,000 Da). Its large size prevents it from crossing the placental barrier.
Option D: IgA is the main immunoglobulin found in mucosal secretions, including saliva, tears, and breast milk (especially colostrum). It provides mucosal protection but does not cross the placenta.

Visual 1: Diagram - The provided diagram effectively shows the selective transport of IgG across the placenta via FcRn receptors, while IgM and IgA are blocked.
Visual 2: Infographic - A comparative chart showing the different structures (monomer, dimer, pentamer) and relative sizes of IgG, IgA, and IgM to visually explain why only IgG can cross the placenta.

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.
Nurses should educate expectant mothers on the importance of third-trimester vaccinations (like Tdap) to maximize the transfer of protective IgG antibodies to the fetus.
Understanding IgG transfer is crucial in managing Rh-negative mothers. The administration of Rho(D) immune globulin prevents the mother from forming anti-D IgG antibodies that could harm an Rh-positive fetus.
Nurses in neonatal units must be aware that preterm infants have lower levels of maternal IgG, placing them at a higher risk for infections. This necessitates strict infection control measures and close monitoring.

How to Approach the Question

First, identify the core concept of the question, which is the transfer of substances across the placenta.
The keyword is 'immunoglobulin'. Recall the different classes of immunoglobulins (IgG, IgA, IgM, IgE, IgD) and their basic properties.
Focus on the unique characteristics of each class related to size and function. Remember that size is a major factor in crossing the placenta.
Recall or deduce that IgG is the smallest major immunoglobulin and is specifically adapted for placental transfer to provide passive immunity.
Eliminate the other options: IgM is famously large (a pentamer), IgA is associated with secretions (like breast milk), and IgE is linked to allergies.

Concept Tested & Keywords

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