CD40LG and systemic lupus erythematosus: Significantly lower IgM anti-PC but not anti-MDA was seen in MCTD compared to HC. No significant difference in levels of anti-PC nor anti-MDA was identified between disease groups, but low levels of IgM were more prevalent in MCTD, SLE, SjS, SSc and UCTD, more for anti-PC than anti-MDA.