Which antibody crosses the placenta?

Appeared in: AIIMS Raipur - 2017 (Shift-3)

Explanation

Immunoglobulin G (IgG) is the only antibody class that is actively transported across the placenta, a process that begins around 16 weeks of gestation and increases significantly during the third trimester.
This transfer provides the fetus and newborn with passive immunity, offering protection against infections to which the mother has immunity.
Due to its relatively smaller molecular weight and specific Fc receptors on placental cells, IgG can efficiently move from maternal to fetal circulation.
The antibodies transferred are a reflection of the mother's immunological history, protecting the baby from common pathogens after birth.

Option A: IgA does not cross the placenta. Its primary role is to provide mucosal immunity.
Option C: IgE does not cross the placenta. It is primarily associated with allergic reactions and defense against parasites.
Option D: IgM does not cross the placenta because of its large pentameric structure.

Visual 1: Diagram: Structure of the five immunoglobulin classes (IgG, IgA, IgM, IgE, IgD), highlighting the monomeric structure of IgG and the larger pentameric structure of IgM.
Visual 2: Infographic: The placental barrier, illustrating the selective transport of IgG from maternal to fetal circulation via Fc receptors.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Maternal-fetal transfer of immunoglobulins as background academic context rather than a clinical decision trigger.
Understanding that only IgG crosses the placenta is vital for neonatal care. It explains why newborns have temporary (passive) immunity to diseases like measles or chickenpox if their mother is immune.
Preterm infants are at a higher risk for infection because the bulk of IgG transfer occurs in the last 4 weeks of pregnancy. They miss this critical window of antibody acquisition.
The presence of IgM in a newborn is a significant diagnostic marker. Since it cannot cross the placenta, elevated IgM levels strongly suggest an intrauterine infection (e.g., rubella, cytomegalovirus, toxoplasmosis).

How to Approach the Question

This is a factual recall question testing fundamental knowledge of immunology.
Identify the keywords: 'antibody', 'crosses placenta'.
Recall the five major classes of immunoglobulins: IgG, IgA, IgM, IgE, IgD.
Associate each immunoglobulin with its key characteristic. A helpful mnemonic is 'IgG Goes across the placenta'.
Consider the structure and function of the other options to confirm they are incorrect: IgA is for mucosal immunity (secretions), IgM is the first responder and is too large, and IgE is for allergies.
Select IgG as the only antibody capable of placental transfer.

Concept Tested & Keywords

Qb0wyPzZu1dn_HVX_ieeDJ

Practise the full AIIMS Raipur - 2017 (Shift-3)

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