Pediatric Immunity—Maternal IgG and the Physiologic Nadir Infants are especially susceptible to bacterial infections when circulating IgG levels are lowest. During which age window is this susceptibility greatest?

Introduction / Context:
Passive maternal IgG is actively transported across the placenta and protects the newborn for several months. Endogenous IgG production in the infant increases gradually. A “physiologic nadir” in total IgG occurs when maternal IgG has decayed but the infant’s own IgG has not fully risen, increasing susceptibility to pyogenic bacterial infections. This question identifies that window.

Given Data / Assumptions:

• Maternal IgG half-life ≈ 21–30 days; substantial decline by 3–6 months.
• Infant IgG synthesis rises through the first year of life.
• Risk of encapsulated bacterial infections increases as maternal antibodies wane.

Concept / Approach:
Locate the overlap between waning maternal IgG and still-developing endogenous production. Most sources place the nadir around 3–6 months, with generally increased susceptibility spanning roughly 3–12 months; hence option C best captures the clinically relevant window among the choices provided.

Step-by-Step Solution:

Verification / Alternative check:
Pediatric immunology references describe the physiologic hypogammaglobulinemia of infancy peaking around 3–6 months, with vulnerability extending into later infancy.

Why Other Options Are Wrong:

• In utero: maternal IgG is high; fetus is protected.
• 0–3 months: antibody levels still relatively protective.
• 12–24 months and beyond: endogenous Ig production is stronger.

Common Pitfalls:
Assuming protection monotonically increases from birth; forgetting the transient nadir.

Final Answer:
3–12 months of age