The basic principle of immunochemical techniques is that a specific antibody will combine with its specific antigen to give an antibody-antigen complex.
Antigens are generally of high molecular weight and are commonly proteins or polysaccharides.
Polypeptides, lipids, nucleic acids and many other materials can also function as antigens. Haptens may be a variety of molecules such as drugs, simple sugars, amino acids, small peptides, phospholipids, or triglycerides. A minimal molecular weight of 8,000-10,000 Daltons, although haptens with molecular weights as low as 200 Da have been used in the presence of a carrier protein. This is usually one to six monosaccharide or amino acid residues on the surface of the antigen. The range of possible binding sites is enormous, with each potential binding site having its own structural properties derived from covalent bonds, ionic bonds and hydrophilic and hydrophobic interactions.