Immune responses of invertebrate animals are mediated through innate mechanisms, among which phagocytosis, encapsulation, production of ROS and antimicrobial peptides. Although polychetes represent an evolutionary interesting group closely related to presumable common ancestor of other celomates, their immune mechanisms still remain scarcely investigated. Here we discuss immune responses of the polychete Arenicola marina, the lugworm. Besides an overview of diversity of celomocytes and cellular responses, we present the synopsis on antimicrobial peptides arenicins: their structure, function and therapeutic potential.