The maternal antibodies that can cross the placenta are?

CGHS SSC - 2018

Obstetrics & Gynaecology

Easy

Appeared in: CGHS SSC - 2018

Explanation

Immunoglobulin G (IgG) is the only antibody class that is actively transported across the placental barrier from mother to fetus.
This transfer provides the newborn with passive immunity, offering protection against infections that the mother has immunity to for the first few months of life.
The transport mechanism involves the neonatal Fc receptor (FcRn) on placental cells and is most prominent during the third trimester of pregnancy.
Due to its relatively small size and specific transport mechanism, IgG is well-suited for this function, unlike the other larger or non-transported immunoglobulins.

Option A: IgA does not cross the placenta. Its structure (often a dimer) prevents placental transfer.
Option C: IgD is present in very small amounts in the blood and primarily functions as an antigen receptor on the surface of B-lymphocytes. It does not cross the placenta.
Option D: IgE is primarily involved in allergic (Type I hypersensitivity) reactions and defense against parasitic infections. 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 immunoglobulins and placental transfer as background academic context rather than a clinical decision trigger.
Understanding that only IgG crosses the placenta is vital for nurses to appreciate the concept of passive immunity in newborns. It explains why newborns are protected against certain infections for the first few months of life.
This knowledge is critical in understanding the pathophysiology of Hemolytic Disease of the Fetus and Newborn (HDFN). If an Rh-negative mother develops anti-D IgG antibodies, these can cross the placenta and attack the red blood cells of an Rh-positive fetus.
What if? If a newborn has elevated levels of IgM antibodies, it does not indicate passive transfer from the mother (as IgM doesn't cross the placenta). Instead, it strongly suggests an active intrauterine infection (e.g., TORCH infections), as the fetus's own immune system produced the IgM.

How to Approach the Question

Identify the core concept of the question, which is the transfer of substances across the placenta.
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE.
Associate each immunoglobulin with its primary function and location. A useful mnemonic is 'GAMED' for the five types.
Focus on the unique properties of each. Remember that IgG is the most abundant in serum and is the only one that 'Goes' across the placenta.
Eliminate the other options based on their known roles: IgA (Secretions), IgE (Allergies), IgD (B-cell receptor). IgM is too large (a pentamer) to cross.
Confirm your answer by selecting IgG, which is responsible for providing passive immunity to the fetus.

Concept Tested & Keywords

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