This article describes a comparative ethnography that investigated how teachers’ immunity would affect personalized vocabulary instruction in IELTS preparation courses. The idea of this paper arose when the basic concept of CLT which has become a fundamental approach of IELTS is off-target for some teachers. So, this study embarked to fill in a gap to describe and compare the personalization level of IELTS vocabulary instruction of five teachers from two ethnographic institutions. Based on the main concepts, this study presented a framework for analysis and categorization of personalized IELTS vocabulary instruction. It also designed using Retrodictive Qualitative Modeling through which the researchers used semi-structured interviews and informal discussions to explore teachers’ archetypes, then, they benefited from classroom observations to see the reasons. The results obtained from the analysis revealed that those teachers with adaptive immunity type had the highest personalized instruction in the cognitive-based level. However, the teacher with maladaptive immunity type, had the highest performance in segmented personalization level which made their students have approximate performance in critical thinking. Also, teachers with productive immunity type had considerable performance in Whole- Person personalization that conducted their students to have proper performance in using critical words and phrases in IELTS. Manuscript profile