CD40LG and infection: Several explanations might account for this finding: (1) IgM-negative children may have been exposed (and potentially infected) but had not yet mounted an IgM response; (2) protective cellular immune responses may have prevented active infection or disease in IgM-negative children; or (3) a true difference in IgG or neutralizing antibody titers exists, but the small sample size (20 families) limited our ability to detect statistical significance.