Haptens are small molecules that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself (in general, only large molecules, infectious agents, or insoluble foreign matter can elicit an immune response in the body).

What do haptens bind?

Haptens are small compounds that penetrate skin and bind to epidermal proteins, generating “modified self” proteins as immunogenic antigens.

Do haptens have the capacity to bind to specific antibodies even if it's no longer bound to its carrier?

The term hapten is derived from the Greek haptein, meaning “to fasten.” Haptens can become tightly fastened to a carrier molecule, most often a protein, by a covalent bond. ... Thus, although the hapten cannot elicit an antibody response on its own, it can bind with antibodies and act as an antigen.