CD40LG and infection: Interestingly, recent observations show that specific IgM levels were significantly increased in both serum and gut mucus upon Aeromonas hydrophila and spring viremia of carp (SVCV) infection, challenging the paradigm that pathogen-specific IgT and IgM responses are strictly compartmentalized in mucosal and systemic immunity, respectively (Mu et al., 2022; Yu et al., 2024).