Non-peptide compounds with molecular weights below 900 daltons are all known as small molecules. Small molecules are common components existing in medicine, food, cosmetics, pesticides, narcotics and research reagents, which also can be found in human bodies as well as the environment. These small molecules are called haptens, which are not immunogenic due to their small molecular weight, but can induce an immune response after conjugation to a large molecular protein. Common haptens includes β-agonists, heavy metals, toxins, mold inhibitors, antibiotic, pigments, pesticides, aquatic fungicides, nutrition inhibitors, food additives, modifying agents, environmental contaminants, hazardous substances produced during processing, and some drugs related to health, such as sulfa, psychotropic drugs, sedative, et al.