All of the following antibodies can cross the placenta, except - - UPPSC 2023

UPPSC 2023

Microbiology

Easy

Appeared in: UPPSC 2023

Explanation

IgM is a very large molecule, structured as a pentamer (five units joined together), which makes it physically too big to pass through the placental barrier.
It is the first antibody produced during an immune response (primary response).
Because IgM cannot cross from mother to fetus, its presence in a newborn's blood indicates that the newborn's own immune system has produced it in response to an infection contracted in the womb (intrauterine infection).

Option A: IgG is the only antibody class that actively crosses the placenta. The question asks for the one that does NOT cross.
Option B: IgA does not cross the placenta. It is primarily a secretory antibody found in mucus, saliva, and breast milk. While technically a correct answer to this flawed 'except' question, IgM is a better choice due to its distinct structural limitation.
Option C: IgE does not cross the placenta. It is associated with allergic reactions and parasitic infections. Like IgA, it is technically a correct answer to this flawed question, but IgM is the classic example.

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 vital for protecting newborns from infections during the first few months of life, as their own immune system is still developing.
A key nursing diagnostic point is testing a newborn's blood for IgM. Finding IgM indicates a congenital infection (like rubella, cytomegalovirus, or toxoplasmosis) because the large IgM molecule cannot come from the mother and must have been produced by the fetus.
What if? If a pregnant woman is Rh-negative and the fetus is Rh-positive, her body can produce anti-Rh IgG antibodies. These IgG antibodies can cross the placenta and cause hemolytic disease of the newborn in a subsequent pregnancy. This demonstrates the clinical downside of IgG's placental transfer.

How to Approach the Question

First, identify the keyword 'except'. This tells you that three of the options are similar in some way, and one is the outlier. The question asks for the outlier.
The core concept is 'antibodies that can cross the placenta'. So, you are looking for the one antibody that CANNOT cross the placenta.
Recall the basic properties of the main immunoglobulin classes. Remember the mnemonic 'GAMED' for the five types.
Focus on the unique function of IgG: it is the only antibody that crosses the placenta to provide passive immunity.
This means IgA, IgE, and IgM do not cross. Since the question requires a single answer, consider the most distinct reason. IgM's large pentameric structure is the most definitive reason it cannot cross.
Therefore, select IgM as the correct exception.

Concept Tested & Keywords

Concept Tested: Placental transfer of immunoglobulins and passive immunity.
Stem keywords: antibodies, cross the placenta
Lead-in keywords: except
Negative lead-in flag: The question asks for the exception among the given options.

QomZZG6FOpbo2mvAbRWBWy

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

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

E6 Microbiology Applied AnjuDhir 3e Part 2 p. 94-96