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

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

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Disease Associated with Immune System
To treat HIV infections using drugs, the major problem is that

the virus particles with altered (mutant) proteases arise
the drugs that are good inhibitors cannot by synthesized
the drugs interfere with normal digestion
the drugs are rapidly degraded

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