– 4-Octylphenol is an endocrine disruptor with gender-specific effects on male germ cells, significantly reducing the mitotic index and the number of spermatogonia. It also causes inflammatory damage in the gills of carp by activating the complement system through the C3a/C3a receptor (C3a/C3aR) axis and C5a/C5a receptor 1 (C5a/C5aR1) axis. Furthermore, 4-Octylphenol induces immune suppression by disrupting the balance between helper T (Th) cells 1/Th2 and regulatory T (Treg)/Th17 cells, and triggers inflammatory damage through the Toll-like receptor 7 (Toll-like Receptor (TLR))/inhibitor κBα/nuclear factor κB (TLR7/IκBα/NF-κB) pathway.