Plants and animals must avoid becoming a free meal to microbes, which vastly outnumber eukaryotic life in both quantity and diversity. Adaptive immunity in the strict sense, whereby the host creates an immunological memory after exposure to a pathogen, is limited to vertebrates. Both plants and animals (including insects and mammals) have an innate immune system, which helps protect hosts from the majority of microbes they encounter during their lifetime. Plant and animal innate immune systems recognize an overlapping set of conserved microbe-associated molecular patterns (MAMPs). This observation suggests that the innate immune system in plants and animals may have been derived from a common ancestor. However, the majority of data indicate that innate immunity has arisen independently in plants and animals and that functional overlap is the result of convergent evolution: confronted with the same problem, and given the same molecular tools, plants and animals have independently derived similar solutions. This review discusses the functional and mechanistic details of the innate immune system in plants and animals including receptor-mediated immunity, endolysosomal immunity, and the interplay of the innate immune system and host-associated microbial communities.

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