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?
**Core Concept**
IgG subclasses have distinct functional roles in immune defense, with IgG2 being specifically critical for opsonization and phagocytosis of encapsulated bacteria such as *Streptococcus pneumoniae* and *Haemophilus influenzae*. Deficiency in IgG2 leads to impaired clearance of these organisms, increasing susceptibility to recurrent sinopulmonary infections.
**Why the Correct Answer is Right**
IgG2 is the primary antibody subclass responsible for neutralizing and opsonizing encapsulated bacteria. It binds effectively to the polysaccharide capsules of these organisms, enabling phagocytosis by neutrophils and macrophages. In a child with repeated infections due to encapsulated pathogens, a deficiency in IgG2 directly impairs this defense mechanism. This is a hallmark of selective IgG2 deficiency, a known cause of recurrent bacterial infections in early childhood.
**Why Each Wrong Option is Incorrect**
Option A: IgG4 is involved in immune complex formation and is not primarily responsible for clearing encapsulated bacteria. Its deficiency is rare and not linked to sinopulmonary infections.
Option B: IgG3 is a potent opsonin but is more effective against small, non-encapsulated bacteria and is not the main mediator for encapsulated organisms.
Option D: IgG1 is effective against both encapsulated and non-encapsulated bacteria and plays a broad role in antibody-mediated immunity, but it is not specifically dominant in clearing encapsulated pathogens.
**Clinical Pearl / High-Yield Fact**
In children with recurrent sinopulmonary infections due to encapsulated bacteria, **IgG2 deficiency** is the most common immunoglobulin deficiency and should be suspected when there's a history of early-onset pneumonia or otitis media. A simple serum immunoglobulin profile with IgG subclass analysis is essential for diagnosis.
✓ Correct Answer: C. IgG2