CD40LG and infection: IgG ceases to proliferate as it loses stimulation from antigen–antibody complexes, resulting in its level being maintained at a relatively low state, as depicted in Figure 4A. However, during the second infection, the initial concentration of IgG is non-zero, so its growth mainly comes from the stimulus of IgG–virus complexes for its renewed production, rather than primarily from the conversion of IgM.