Haptens can interact with the product of the immune response, but
can't elicit the response. If haptens represent free epitopes, B-cells
and T-cells can interact with them. Why are haptens unable to induce
an immune response?

Hey ruggedbug-ga, how are you doing?

In order to induce an immune response they must be attached to carrier
molecules that form a strong covalent bond. Low molecular weight
substances that are able to produce an allergic response are known as
haptens.

REFERENCE
David H. Priest, MD: An Examination of Penicillin Allergy
Wake Forest University School of Medicine
Internal Medicine Residency Program
http://intmedweb.wfubmc.edu/grand_rounds/2001/penicillin.html

Best wishes,