Question:
A child suffering from recurrent chronic infections with encapsulated bacteria has a deficiency of which IgG subclass?
A child suffering from recurrent chronic infections with encapsulated bacteria has a deficiency of which IgG subclass?
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Which IgG subclass answers polysaccharide capsule antigens?
Updated On: Jun 23, 2026
- IgG1
- IgG2
- IgG3
- IgG4
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The Correct Option is B
Solution and Explanation
Step 1: Encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae) are protected by a polysaccharide capsule. Defence against them requires antibodies directed at this polysaccharide.
Step 2: Anti-polysaccharide antibody responses are mediated predominantly by the IgG2 subclass. So a deficiency of IgG2 impairs opsonization of capsular organisms.
Step 3: This is why selective IgG2 deficiency presents as recurrent, chronic infections with encapsulated bacteria, especially of the respiratory tract.
Step 4: IgG1 and IgG3 mainly target protein antigens and IgG4 is associated with chronic antigen exposure and allergy, so they do not explain susceptibility to capsulated bacteria. The answer is IgG2.
Step 2: Anti-polysaccharide antibody responses are mediated predominantly by the IgG2 subclass. So a deficiency of IgG2 impairs opsonization of capsular organisms.
Step 3: This is why selective IgG2 deficiency presents as recurrent, chronic infections with encapsulated bacteria, especially of the respiratory tract.
Step 4: IgG1 and IgG3 mainly target protein antigens and IgG4 is associated with chronic antigen exposure and allergy, so they do not explain susceptibility to capsulated bacteria. The answer is IgG2.
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