A child is suffering from recurrent chronic infections with encapsulated bacteria; Which subclass of IgG does the child has deficiency?
IgG has four subclasses: IgG1, IgG2, IgG3, and IgG4. Each has different functions. IgG1 and IgG3 are good at fixing complement and opsonization, which help in fighting encapsulated bacteria. IgG2 is important for polysaccharide antigens, like those in encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae. So if a child has a deficiency in IgG2, they might not be able to mount an effective immune response against these bacteria, leading to recurrent infections.
Now, the question is about which subclass is deficient. The options were A, B, C, D. Since the correct answer is C, I need to figure out which subclass corresponds to option C. Wait, maybe the options were IgG1, IgG2, IgG3, IgG4. If the correct answer is IgG2, then option C would be the right one. Let me check standard MCQ patterns. Usually, IgG2 deficiency is associated with recurrent infections in children, especially with encapsulated organisms. So the correct answer is IgG2. Therefore, if option C is IgG2, that's the answer.
For the explanation, the core concept is IgG subclasses and their roles in immunity against encapsulated bacteria. The correct answer is IgG2 because it's crucial for polysaccharide responses. The other options are incorrect because IgG1 and IgG3 are more effective in complement fixation and opsonization. IgG4 doesn't play a significant role in this context. The clinical pearl is that IgG2 deficiency is a common cause of recurrent infections in children. Let me structure this into the required sections, making sure to mention the exact subclasses and their functions.
**Core Concept**
IgG subclasses (IgG1, IgG2, IgG3, IgG4) differ in their roles against pathogens. IgG2 is critical for immune responses to polysaccharide antigens in encapsulated bacteria like *Streptococcus pneumoniae* and *Haemophilus influenzae*, which are common causes of recurrent infections in immunodeficiencies.
**Why the Correct Answer is Right**
IgG2 deficiency impairs the body’s ability to produce antibodies against polysaccharide capsules of bacteria. These capsules evade phagocytosis unless opsonized by IgG2. The absence of IgG2 leads to defective opsonization and complement activation, resulting in chronic infections. This deficiency is the most common cause of selective IgG subclass deficiency in children.
**Why Each Wrong Option is Incorrect**
**Option A:** IgG1 deficiency is rare and typically does not cause recurrent infections, as IgG1 targets protein antigens rather than polysaccharides.
**Option B:** IgG3 deficiency is uncommon and primarily affects responses to protein antigens, not encapsulated bacteria.
**Option D:** IgG4 deficiency is clinically insignificant; IgG4 has minimal role in anti-infective immunity.
**Clinical Pearl / High-Y