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

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Disease Associated with Immune System
Bone marrow given to an infant with SCID must

be irradiated to eliminate GVHD
come from one of the child's parents
contain mature T cells that can begin making immune responses immediately
come from a donor that shares some MHC alleles with the recipient

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