CD40LG and infection: By contrast, in SARS associated infections, nearly all SARS patients show virus-specific IgM and IgG antibody by week 3, which began to decrease in the 3–4 week after illness onset, and IgM remained at a low level in a few weeks, while the concentrations of IgG antibody that is 4∼8 times higher than that of IgM antibody, reaching its peak at 20 days after infection, and then maintained in a high level for a long time (Woo et al., 2004; Mo et al., 2005).