Immunology—X-Linked Hyper-IgM (CD40L Deficiency) In X-linked hyper IgM syndrome caused by defective CD40L on T helper cells, which specific immune interaction is PREVENTED?
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AActivation of B cells by T-independent antigens such as bacterial polysaccharides
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BProvision of co-stimulation by B cells required for Th2 activation
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CProvision of CD40L-dependent co-stimulation by Th cells that is required for B-cell isotype switching
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DAny and all B-cell proliferation events regardless of stimulus
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EFormation of IgM pentamers in the serum
Answer
Correct Answer: Provision of CD40L-dependent co-stimulation by Th cells that is required for B-cell isotype switching
Explanation
Introduction / Context:X-linked hyper IgM syndrome (HIGM) is classically due to mutations in CD40 ligand (CD40L, CD154) on activated CD4+ T cells. CD40L is essential for cognate T–B interactions that drive isotype (class) switching and germinal center formation. Patients have high/normal IgM but very low IgG, IgA, and IgE, with susceptibility to opportunistic infections. This item asks which precise immune event is blocked when CD40L is defective.
Given Data / Assumptions:
- CD40L on Th cells binds CD40 on B cells and dendritic cells.
- CD40 signaling in B cells is necessary for class switching and germinal center reactions.
- T-independent antigens can activate B cells without T-cell help.
Concept / Approach:Identify the CD40L-dependent step: T-cell help to B cells for isotype switching. Without CD40L–CD40 engagement, activation-induced cytidine deaminase (AID)–dependent class switching fails, trapping responses in the IgM isotype and impairing affinity maturation.
Step-by-Step Solution:
Step 1: Map the interaction: Th cell CD40L ↔ B-cell CD40 → signals for switching.Step 2: Determine outcomes: absent switching → low IgG/IgA/IgE; often normal/high IgM.Step 3: Select the option that directly names the missing co-stimulatory help for isotype switching.Verification / Alternative check:Clinical phenotypes and in vitro studies of CD40L deficiency show failure to form germinal centers and to switch isotypes despite presence of B cells.
Why Other Options Are Wrong:
- A: T-independent activation does not require CD40L.
- B: Inverting the direction of co-stimulation; B → Th co-stimulation is not the central defect in HIGM.
- D: B-cell proliferation can occur via other signals (e.g., TLRs, BCR with cytokines), though germinal center proliferation is impaired.
- E: IgM assembly is intact; the problem is switching away from IgM.
Common Pitfalls:Assuming all B-cell activation requires T-cell help; confusing general proliferation with the specific requirement for CD40L in class switching.
Final Answer:Provision of CD40L-dependent co-stimulation by Th cells that is required for B-cell isotype switching