↑ IgM against H. alvei, P. aeruginosa, M. morganii and P. putida in MDD. ↑ IgA against H. alvei, P. aeruginosa, M. morganii, K. pneumonia in MDD. ↑ Peak IgM and IgA responses in MDD. ↑ Peak IgM responses in chronic MDD versus non-chronic MDD. Significant differences in IgM responses between patients with chronic MDD (duration > 2 years) and controls. Here, CD79A is linked to major depressive disorder.