Here we show that DENV-specific IgM antibodies likely play a role in neutralization during primary DENV infections and show a correlation between serum avidity and neutralization capacity in secondary DENV infections, with greater avidity to a previously infecting DENV serotype as compared to the current infecting DENV serotype in the early phases of infection, switching over time to the opposite situation. The gene discussed is CD40LG; the disease is infection.