Animals and plants use conceptually similar immune systems to detect microbes and activate defenses. In both lineages, pattern recognition receptors (PRRs) perceive pathogen- or microbe-associated molecular patterns (PAMPs or MAMPs) and host-derived damage-associated molecular patterns (DAMPs). Whereas animals rely on specialized immune cells, plants harness individual cells expressing diverse PRRs. In the apoplast, which lies outside the plasma membrane and includes the cell wall, PRRs detect immunogenic fragments released from microbial proteins by plant hydrolytic enzymes. Conversely, pathogen-secreted enzymes can degrade constituents of the apoplast, generating DAMPs that activate immunity. In three new Science papers, researchers report expansions of this surveillance, revealing receptor architectures and mechanisms that enable plants to discriminate immunogenic carbohydrates. The findings illustrate how ligand specificity and receptor assembly confine immune activation to the context of microbial challenge. Learn more in a new #SciencePerspective: scim.ag/4ib9BRC