Teacher factors encompass the teachers’ cognitive and affective characteristics that are likely to affect their instructional efficacy. Language teacher immunity is one of the affective factors that determines the teachers’ psychological well-being in their settings. The present study strived to itemize the factors in novice and experienced EFL instructors’ teacher immunity philosophy. To this end, first, the researchers used convenience sampling for selecting 62 EFL teachers including 38 novice and 24 experienced teachers at ten prominent language institutes in Tabriz (Iran) as the participants. Second, they conducted 30-minute interviews with the participants for determining the factors that influenced their TI philosophy using a researcher-developed semi-structured interview protocol. Lastly, they used thematic analysis in order to particularize the main factors in their teacher immunity philosophy. The findings of the study indicated that while the novice teachers considered external support as the main factor in their philosophy, the experienced teachers regarded their internal strength as the decisive factor that influenced their teacher immunity philosophy. The findings may have practical implications for teacher education courses in foreign language contexts.