The term "hapten" does not belong in the list because it is not a complete antigen. Haptens are small molecules that can bind to antibodies, but they are unable to induce an immune response on their own.

A hapten is a small molecule that can elicit an immune response only when conjugated with a large carrier such as a protein. Typical haptens include drugs, urushiol, quinone, steroids, etc. This is what I read on Creative Biolabs.

hapten

A hapten is a small molecule that can elicit an immune response only when conjugated with a large carrier such as a protein. Typical haptens include drugs, urushiol, quinone, steroids, etc. This is what I read on Creative Biolabs.

Because the molecule is too small to cross-link with immune-cells