Due to the novelty of the concept of language teacher immunity, scant research attention has been dedicated to exploring its personality-based and psychological correlates in language teaching research. To expand the line of research investigating the contributing factors to language teachers' immunity, the present study, informed by positive psychology, aimed to test a structural model of English as a Foreign Language (EFL) teachers' L2 grit, work engagement, and immunity. The participants were 369 Iranian EFL teachers who completed three electronic surveys. The results of Structural Equation Modelling (SEM) indicated that EFL teachers' immunity was significantly influenced by their L2 grit and work engagement. In addition, EFL teachers' L2 grit was found to be a stronger predictor of their immunity than work engagement. The findings revealed that EFL teachers with higher levels of work engagement and L2 grit tended to be more immune to professional challenges, which subsequently helped them overcome stressors resiliently and remain committed to the profession. The results highlight the significance of applying positive psychology in the field of foreign language teacher education and add to our understanding of the antecedents of language teachers' immunity in EFL contexts. Finally, suggestions for future research are offered.