Disease Associated with Immune System
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

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
activation of B cells by T-independent antigens
failure of B cells to provide co-stimulation for Th2 activation

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