X-linked hyper IgM syndrome, resulting in high levels of serum IgM and low levels of serum IgG, is caused by a defect in CD40L expression. The specific immune event that would be prevented by a defective CD40L would be

• activation of B cells by T-independent antigens
• failure of B cells to provide co-stimulation for Th2 activation
• failure of Th2 cells to provide co-stimulation for B cell isotype switching
• failure of Th2 cells to provide co-stimulation for B cell proliferation