Language teacher immunity has recently been introduced as a protective system that helps language teachers maintain their professional development despite distur-bances within classroom settings. However, investigating how to develop in-service language teachers\\\' immunity, especially in English as a Foreign Language (EFL) contexts, has received scant research attention. Similarly, post-induction mentoring of in-service EFL teachers has remained less explored. The current mixed-meth-ods study aimed to identify dominant immunity archetypes among in-service EFL teachers (N=390) and to explore whether post-induction mentoring can facilitate the immunity development of in-service EFL teachers (N=6) in the Iranian context. Two-step cluster analysis revealed the prevalence of two immunity clusters (vision-ary archetype and fossilized archetype) among in-service EFL teachers. Thematic analysis based on unstructured observations of implemented mentoring sessions and following qualitative interviews yielded four major themes about the significance of post-induction mentoring for developing in-service EFL teachers\\\' productive immu-nity through developing their resilience, encouraging them to promote innovation in their pedagogical practices, to provide their learners with emotional support, and to cope with professional hurdles. Finally, suggestions for future research are offered.

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