CD40LG and rheumatoid arthritis: The abundance of all four IgG isotypes, but not of the IgM isotype, was higher in the RF(−)/anti-CCP(+) RA group than in the RF(−)/anti-CCP(−) control group, and the abundance of IgG2 isotype was higher in the RF(−)/anti-CCP(+) RA group than in the RF(−)/anti-CCP(−) RA group.