A 3-year-old boy has recurrent pyogenic infections, severe Pneumocystis jirovecii pneumonia, and elevated serum IgM with markedly decreased IgG, IgA, and IgE. Which molecular interaction is defective in this patient?
- A CD40L on T cells binding to CD40 on B cells ✓
- B B7 on B cells binding to CD28 on T cells
- C MHC class II on B cells binding to TCR on T cells
- D ICAM-1 on B cells binding to LFA-1 on T cells
Correct answer: A. CD40L on T cells binding to CD40 on B cells
Explanation
X-linked Hyper-IgM syndrome is caused by a mutation in the CD40 ligand gene on T cells. Without CD40L-CD40 interaction, B cells cannot undergo isotype switching, remaining stuck producing IgM. They also fail to form germinal centers properly. B7-CD28 is for co-stimulation, and MHC-TCR is for antigen presentation.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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