CD40LG and infection: However, only 25% of IgM positive sera to CHIKV showed high avidity to the rE2 protein, which could be related to the preferential binding of IgM on epitopes surface of E1-E2 glycoproteins, rather than to the individual E2 [37] or to the relative immaturity of the first produced antibodies in the acute phase of the infection by CHIKV [38].