The plant immune system is a two-branched innate defense mechanism that responds to pathogens and non-pathogens. The first branch recognizes and responds to microbial-associated molecular patterns (MAMPs) through pattern recognition receptors (PRRs), while the second branch responds to pathogen virulence factors, either directly or indirectly through their effects on host targets. These systems provide insights into molecular recognition, cell biology, and evolution across biological kingdoms. The plant immune system can be represented as a four-phased 'zigzag' model, where PAMP-triggered immunity (PTI) is followed by effector-triggered susceptibility (ETS), then effector-triggered immunity (ETI), and finally, pathogens evolve to avoid ETI. The recognition of pathogen effectors by NB-LRR proteins is crucial for ETI, and the evolution of these proteins is driven by the co-evolution with pathogen effectors. Understanding the plant immune system is essential for improving crop resistance to diseases.The plant immune system is a two-branched innate defense mechanism that responds to pathogens and non-pathogens. The first branch recognizes and responds to microbial-associated molecular patterns (MAMPs) through pattern recognition receptors (PRRs), while the second branch responds to pathogen virulence factors, either directly or indirectly through their effects on host targets. These systems provide insights into molecular recognition, cell biology, and evolution across biological kingdoms. The plant immune system can be represented as a four-phased 'zigzag' model, where PAMP-triggered immunity (PTI) is followed by effector-triggered susceptibility (ETS), then effector-triggered immunity (ETI), and finally, pathogens evolve to avoid ETI. The recognition of pathogen effectors by NB-LRR proteins is crucial for ETI, and the evolution of these proteins is driven by the co-evolution with pathogen effectors. Understanding the plant immune system is essential for improving crop resistance to diseases.