Final answer:
Haptens are small molecules that become antigenic when attached to a carrier protein in conjugate vaccines. They allow the immune system to recognize and mount an immune response against polysaccharide antigens in certain pathogens.
Step-by-step explanation:
Haptens are small molecules that are not antigenic by themselves, but become antigenic when they attach to a larger carrier molecule, usually a protein. In the case of conjugate vaccines, haptens are attached to a carrier protein to produce a conjugate antigen. The immune response to the conjugate antigen results in the production of antibodies specific to the hapten. Haptens play a crucial role in conjugate vaccines by allowing the immune system to recognize and mount an immune response against the polysaccharide antigens present in certain pathogens.