A child present with recurrent episodes of sinopulmonary infection by bacteria with polysachride rich capsule. Deficiency of which of the following immunoglobulin subclasses should be specifically investigated –
**Core Concept**
The child's presentation with recurrent sinopulmonary infections by bacteria with polysaccharide-rich capsules suggests an issue with the immune system's ability to recognize and respond to these encapsulated pathogens. This is a classic feature of antibody-mediated immunity, specifically involving the complement system and opsonization.
**Why the Correct Answer is Right**
The correct answer is related to the immunoglobulin subclass responsible for responding to polysaccharide antigens. The primary immunoglobulin subclass involved in this response is IgG2. IgG2 antibodies are essential for recognizing and opsonizing encapsulated bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis, which have polysaccharide-rich capsules. The complement system, specifically the alternative pathway, is activated by the binding of IgG2 antibodies to these capsules, leading to bacterial lysis and clearance.
**Why Each Wrong Option is Incorrect**
**Option A:** IgG1 is involved in responses to protein antigens and is not the primary immunoglobulin subclass responsible for responding to polysaccharide-rich capsules.
**Option B:** IgG4 is involved in responses to protein antigens and is less important in the context of polysaccharide-rich capsules.
**Option C:** IgA is primarily involved in mucosal immunity and is not the primary immunoglobulin subclass responsible for responding to polysaccharide-rich capsules.
**Clinical Pearl / High-Yield Fact**
In patients with recurrent sinopulmonary infections, especially those caused by encapsulated bacteria, a deficiency in IgG2 should be investigated. This can be done through serum immunoglobulin subclass analysis or through functional assays, such as the Quellung test.
**Correct Answer:** C.