A child has a history of recurrent infections with organisms having polysaccharide antigens (i.e., Streptococcus pneumoniae and Haemophilus influenzae). This susceptibility can be explained by a deficiency of ?
**Core Concept**
The question is testing the understanding of the immunological basis of susceptibility to polysaccharide-encapsulated bacteria. The correct answer is related to a specific antibody isotype that plays a crucial role in providing immunity against these pathogens. **Complement system** and **antibody-mediated immunity** are also relevant concepts in this context.
**Why the Correct Answer is Right**
The child's susceptibility to recurrent infections with organisms having polysaccharide antigens, such as Streptococcus pneumoniae and Haemophilus influenzae, is due to a deficiency in IgG2 antibodies. **IgG2 antibodies** are primarily responsible for providing immunity against polysaccharide-encapsulated bacteria. This is because the J chain, which is necessary for the formation of IgG2 antibodies, is not present or is present in low amounts in individuals with this deficiency. As a result, the child is unable to mount an effective immune response against these pathogens.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because IgA antibodies are primarily involved in providing immunity against mucosal pathogens, not polysaccharide-encapsulated bacteria.
* **Option B:** This option is incorrect because IgM antibodies are primarily involved in providing immediate, short-term immunity against infections, not long-term immunity against polysaccharide-encapsulated bacteria.
* **Option C:** This option is incorrect because IgE antibodies are primarily involved in providing immunity against parasites and in the pathogenesis of allergic reactions, not against polysaccharide-encapsulated bacteria.
**Clinical Pearl / High-Yield Fact**
Individuals with a deficiency in IgG2 antibodies are at increased risk of developing invasive infections with polysaccharide-encapsulated bacteria, such as Streptococcus pneumoniae and Haemophilus influenzae. This is because their immune system is unable to mount an effective response against these pathogens.
**Correct Answer: D. IgG2 deficiency.**