**Core Concept**
Recurrent sinopulmonary infections by encapsulated bacteria are often seen in individuals with defects in their humoral immune response, particularly in the production of specific immunoglobulin subclasses.

**Why the Correct Answer is Right**
The correct answer is related to the production of antibodies against polysaccharide antigens, which are primarily mediated by immunoglobulin G2 (IgG2) subclass. This subclass is crucial for providing protection against encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. Individuals with IgG2 deficiency have impaired antibody responses to polysaccharide antigens, leading to recurrent infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** IgG1 is the most abundant subclass of IgG and plays a significant role in neutralizing toxins and viruses, but it is not primarily responsible for protecting against encapsulated bacteria.
* **Option B:** IgE is involved in allergic reactions and parasite immunity, not in protecting against encapsulated bacteria.
* **Option C:** IgM is the first antibody produced in response to an infection and is important for activating the complement system, but it is not the primary subclass responsible for providing long-term protection against encapsulated bacteria.

**Clinical Pearl / High-Yield Fact**
IgG2 deficiency should be suspected in patients with recurrent sinopulmonary infections and impaired antibody responses to polysaccharide antigens, such as those seen in individuals with chronic otitis media, sinusitis, or bronchopneumonia.

**Correct Answer:** C.