Many immune systems—prokaryotic and eukaryotic—appear to have recruited parts from mobile genetic elements (MGEs) they now defend against. In this chapter, I advance an integrative hypothesis that defense pathways across life emerged repeatedly through co-option of MGEs (viruses, transposons, and viroid-like RNAs), with roots extending to protocells in the ancient RNA world. In this view, early ribozyme communities inevitably faced parasitic replicators; trans-cleaving ribozymes and superinfection exclusion-like interference provide plausible models for primordial immunity. Random ligation among catalytic RNAs could have produced a multi-ribozyme “protogenome” that domesticated parasites by coupling their transmission to host replication. This coupling could then have enabled genome expansion and, ultimately, a transition to more stable DNA heredity. In prokaryotes, defense islands, restriction–modification systems, and CRISPR-Cas exemplify how mobile modules are domesticated into immunity. In eukaryotes, RNA silencing (siRNA/piRNA), interferon circuitry shaped by transposable-element-derived regulatory elements, and endogenous retroviral co-options (including Env-mediated receptor interference) illustrate the same principle. In vertebrates, adaptive immunity itself derives from a transposon domestication event, in which a Transib-related recombinase became the engine of antibody and T-cell receptor diversification. I synthesize these lines into a continuous evolutionary narrative: host-parasite conflict repeatedly turns “weapons into shields,” driving innovation from pre-LUCA (Last Universal Common Ancestor) stages to modern immunity. I close with testable experimental proposals to evaluate key steps in this co-option framework. This perspective reframes immunity as a creative legacy of genetic conflict.