Hello,
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Are you talking about courses or IITs branches.
If you are talking about courses then,
No, a commerce student is not eligible for IIT courses.
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If you are talking about Branches then  each IIT Eligiblity criteria may vary. Like recently IIT Delhi and Madras started giving admission for commerce student.
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You haven't mentioned any particular IIT state. But remember that your percentage in 10+2 must be above 80%.