Antibody which can cross the placenta is

UPPSC 2022

Microbiology

Easy

Appeared in: UPPSC 2022

Explanation

IgG is the only class of immunoglobulin that is actively transported across the placental barrier.
This transfer provides the fetus and newborn with passive natural immunity against a wide range of pathogens to which the mother is immune.
The structure of IgG is a monomer, which is small enough to facilitate this transport, unlike larger immunoglobulins.
This maternal IgG protects the infant for the first few months of life until its own immune system matures.

Option B: IgM is a large pentamer (a molecule composed of five smaller units), which is too large to pass through the placental barrier.
Option C: IgE is primarily associated with allergic reactions (Type I hypersensitivity) and defense against helminthic parasites. It does not cross the placenta.
Option D: IgA is the primary secretory antibody and is found in mucosal linings, tears, saliva, and breast milk. 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 Placental transfer and functions of different immunoglobulin classes as background academic context rather than a clinical decision trigger.
The transfer of maternal IgG is the cornerstone of neonatal immunity, protecting infants from common infections like measles, mumps, and rubella for the first 6-12 months if the mother is immune.
A key nursing diagnostic point: if a newborn's blood test reveals the presence of IgM antibodies, it signifies an active infection that occurred in utero (congenital infection), as these large antibodies cannot come from the mother. This requires immediate investigation for pathogens like TORCH (Toxoplasmosis, Other, Rubella, Cytomegalovirus, Herpes).
Nurses play a vital role in educating new mothers about the benefits of breastfeeding, as it provides the infant with secretory IgA from colostrum, which protects the baby's gut from infections.

How to Approach the Question

This is a factual recall question testing knowledge of immunology.
First, identify the core concept of the question: which antibody can cross the placenta.
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgE, and IgD (mnemonic: GAMED).
Associate each class with its primary function and structure. Remember that structure often dictates function.
Specifically, recall that IgG is the antibody responsible for long-term immunity and is small (a monomer), allowing it to pass through the placenta to the fetus.
Eliminate the other options based on their known properties: IgM is too large (pentamer), IgA is secretory, and IgE is for allergies. This leaves IgG as the only possible answer.

Concept Tested & Keywords

Concept Tested: Placental transfer and functions of different immunoglobulin classes.
Stem keywords: Antibody, cross, placenta
Lead-in keywords: is

q84PWN_X57Zl-WVgGvdfw

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

E6 Microbiology Essentials Sastry 4e Part 1 p. 150-152

E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart B (pp 240-463 of 478) p. 92-94