Collective defences (pale grey, dotted line) of a group comprises all individual defences (elipse) of the group members and their interaction (arrows). Individual defences are composed of anti-parasite behaviours (B, dark grey) and physiological immune systems that can contain either only the innate (I) immune component (e.g. invertebrates) or also the acquired (A) immune component (e.g. vertebrates). The single immune modules interact with each other during anti-parasite defence and thus shape each others evolution (adapted from Cremer & Sixt 2009 Phil Trans Roy Soc B)