To conclude this section, specialization and attendant host specificity is a central, emergent property of parasitism and has multiple underlying determinants, involving both encounter and compatibility filters. The ubiquity of host defenses and the evidence that they often eliminate novel parasites argue that host immune systems play a critical role in limiting parasite host www.frontiersin.org Table 3 | Examples of colonizing parasites, or parasites placed in novel hosts, that are killed or limited by immune responses.