Substance that crosses the placental barrier is?

Appeared in: DSSSB Public Health Nurse - 2015

Explanation

Immunoglobulin G (IgG) is the only antibody class that is actively transported across the placental barrier from the mother to the fetus.
This transfer provides the fetus and newborn with passive immunity, protecting them against infections to which the mother has immunity.
The process begins around 16 weeks of gestation and peaks in the third trimester, which is why preterm infants are more vulnerable to infections due to lower levels of maternal IgG.
This passive immunity is temporary and wanes over the first few months of life as the infant's own immune system begins to produce its own antibodies.

Option B: IgA does not cross the placental barrier. It is the primary immunoglobulin found in mucosal secretions.
Option C: IgM is a large pentameric molecule, which is too big to cross the placenta.
Option D: IgE is primarily involved in allergic reactions and defense against parasites and 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 of immunoglobulins and passive immunity as background academic context rather than a clinical decision trigger.
The transfer of maternal IgG is the principle behind passive immunity in newborns, protecting them from diseases like measles, tetanus, and diphtheria if the mother is vaccinated or has had the disease.
A key diagnostic point for nurses is that the presence of IgM antibodies in a newborn's blood is a strong indicator of a congenital infection (e.g., rubella, cytomegalovirus, toxoplasmosis), as the fetus must have produced these antibodies itself.
Nurses should educate new mothers that breastfeeding provides additional immune protection through secretory IgA in colostrum and milk, which protects the infant's gut and respiratory system.

How to Approach the Question

Identify the core concept of the question: the transfer of substances across the placenta.
Recognize that the question is specifically asking about immunoglobulins (antibodies).
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgE, and IgD.
Consider the unique properties of each class, especially size and function.
Remember the key fact that only IgG is specifically transported across the placenta to provide passive immunity.
Eliminate the other options: IgM is too large, IgA is transferred via breast milk for mucosal immunity, and IgE is for allergic responses.

Concept Tested & Keywords

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

E6 Obstetrics Williams p. 29-50

Practise the full DSSSB Public Health Nurse - 2015

Attempt every question from this paper in a timed mock, then review the full solution for each one.