This study claims that the bilingual mental lexicon contains 'lemmas', defined as abstract entries in the mental lexicon about particular lexemes, and such lemmas are in contact in ICS. Lemmas contain information about lexical-conceptual structure, predicate-argument structure and morphological realization patterns of particular lexemes. It further claims that bilingual lemmas are language-specific, and it is the activation of languagespecific lemmas which drives ICS. The ICS instances for the study reveal that bilinguals switch content morphemes from one language into another language's sentential frame because of cross-linguistic lexicalconceptual gaps. Bilinguals do not switch system morphemes because it is the Matrix Language (the host language) which provides the sentential frame, including all system morphemes, and the Embedded Language (the guest language) inserts certain content morphemes as desired into the lexical slots in the ML. The lexicalconceptual approach introduced in this study explores the nature and activity of the bilingual mental lexicon during ICS.