Hapten are small-molecules, that can only become immunogenic when conjugated with a carrier protein. I was wondering if all small-molecules can become haptens (eg. by synthetic conjugation). Given that lymphocytes are filtered by negative selection, I would expect that small-molecules that chemically mimic self-peptides would not work as haptens (at least not in the sense, that they could be detected by antibodies).
Are there any examples of small-molecules that have been tried as haptens but could not generate antibodies ?
Also is there any estimate of the ratio of naturally occuring haptens and synthetic haptens. I was investigating the IEDB (https://www.iedb.org/) as well as SuperHapten (http://bioinformatics.charite.de/superhapten/) but could not find a way to filter these two types of haptens.