CD40LG and major depressive disorder: In 2012, Maes et al [28] showed that bacterial translocation from the gut, or leaky gut, which may be due to systemic inflammation in depression or may be a primary trigger that is associated with the onset of depression, can activate immune cells to elicit selective immunoglobulin A (IgA) and immunoglobulin M (IgM) response—both antibodies that fight and protect against infections.