Do all creatures possess an acquired immune system of some sort?

@article{Rimer2014DoAC,
  title={Do all creatures possess an acquired immune system of some sort?},
  author={Jacob Rimer and Irun R. Cohen and Nir Friedman},
  journal={BioEssays : news and reviews in molecular, cellular and developmental biology},
  year={2014},
  volume={36 3},
  pages={273-81}
}
Recent findings have provided evidence for the existence of non-vertebrate acquired immunity. We survey these findings and propose that all living organisms must express both innate and acquired immunity. This is opposed to the paradigm that only vertebrates manifest the two forms of immune mechanism; other species are thought to use innate immunity alone. We suggest new definitions of innate and acquired immunity, based on whether immune recognition molecules are encoded in the inherited… CONTINUE READING