A 3-year-old child presents with a history of repeated sinopulmonary infections caused by encapsulated organisms. Which of the following is most likely to be deficient in this case?

Correct Answer: D. IgG2

Encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) are opsonized and cleared primarily by IgG2 antibodies. IgG2 is the predominant immunoglobulin against polysaccharide antigens found in bacterial capsules. In children, IgG2 responses develop later than IgG1 (typically after age 2–3 years), explaining why young children are particularly susceptible to encapsulated organism infections. A selective IgG2 deficiency results in recurrent sinopulmonary infections (otitis media, sinusitis, pneumonia) caused by these organisms despite normal total immunoglobulin levels. This is a well-recognized immunodeficiency in Indian pediatric practice, often identified through repeated infections in the 2–5 year age group. The diagnosis is confirmed by demonstrating low IgG2 levels (<40 mg/dL) with normal IgG1, IgG3, and IgG4. Management includes prophylactic antibiotics, vaccination with conjugate vaccines (which bypass the polysaccharide response defect), and sometimes intravenous immunoglobulin (IVIG) in severe cases.

Why the other options are wrong

A. IgG1 — IgG1 is the predominant subclass against protein antigens and is the most abundant IgG subclass overall. IgG1 deficiency causes recurrent infections with protein-coated organisms, not specifically encapsulated organisms. IgG1 develops early in infancy and is not the limiting factor in polysaccharide responses. This is a distractor because IgG1 is quantitatively most important, but functionally irrelevant for capsular immunity. B. IgG4 — IgG4 is the rarest IgG subclass and is involved in responses to allergens and some chronic infections. It has minimal role in opsonization of encapsulated bacteria. IgG4 deficiency does not cause sinopulmonary infections from encapsulated organisms. This option exploits the misconception that rarer subclasses are more clinically significant. C. IgG3 — IgG3 is involved in responses to some protein antigens and has a shorter half-life than other IgG subclasses. While IgG3 can contribute to anti-polysaccharide responses, it is not the primary subclass responsible for opsonization of encapsulated organisms. IgG3 deficiency alone does not typically cause the classic pattern of recurrent sinopulmonary infections seen here.

High-Yield Facts

Mnemonics

IgG2 = Polysaccharide (2 = Sugar) IgG2 responds to polysaccharide (sugar) capsules. IgG1 responds to proteins. Memory: 2 syllables in 'sugar' → IgG2. SHINE for Encapsulated Organisms Streptococcus pneumoniae, Haemophilus influenzae, IgG2 deficiency, Neisseria meningitidis, Encapsulated → all linked to IgG2.

NBE Trap

NBE pairs "repeated infections" with "immunodeficiency" to lure students toward total IgG deficiency or common subclass IgG1. The trap is that IgG1 is quantitatively dominant, making students default to it; however, the specific antigen type (polysaccharide) is the discriminator that points uniquely to IgG2.

Clinical Pearl

In Indian pediatric outpatient settings, a 3–4 year old with recurrent otitis media, sinusitis, or community-acquired pneumonia caused by pneumococcus should raise suspicion for IgG2 deficiency. Most children improve spontaneously by school age, but conjugate vaccine series and prophylactic amoxicillin during winter months are practical interventions in resource-limited settings.

_Reference: Jawetz Melnick & Adelberg's Medical Microbiology Ch. 8 (Immunology); Harrison Principles of Internal Medicine Ch. 372 (Immunodeficiency); OP Ghai Essentials of Pediatrics Ch. 12 (Immunology)_