The classic definition of Antigen is “any foreign substance that elicits an immune response (e.g. the production of specific antibody molecules) when introduced into the tissues of a susceptible animal and is capable of combining with the specific antibodies formed”. 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. Immune responses may also be generated against smaller substances, called “haptens”, if these are chemically coupled to a larger “carrier protein”, such as bovine serum albumin or hemocyanin or other synthetic matrices. Haptens may be a variety of molecules such as drugs, simple sugars, amino acids, small peptides, phospholipids, or triglycerides. Antigens which elicit strong immune responses are said to be “strongly immunogenic”.
• Areas of structural stability and chemical complexity within the molecule
• Randomness of structure (i.e. lack of repeating units)
• 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.
• The ability to be metabolized
• Immunogenic regions which are accessible to the antibody-forming mechanism
• Structural elements that are sufficiently different from the host
The small site on an antigen to which a complementary antibody may specifically bind is called an “epitope”. This is usually one to six monosaccharide or amino acid residues on the surface of the antigen. Because antigen molecules exist in space, the epitope recognized by an antibody may be dependent upon the presence of a specific three-dimensional antigenic conformation (e.g. a unique site formed by the interaction of two native protein subunits), or the epitope may correspond to a simple primary sequence region. Such epitopes are described as “conformational” and “linear”, respectively.
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.
For efficient interaction to occur between the antigen and the antibody, the epitope must be readily available for binding. If the target molecule is denatured, e.g. through fixation, the epitope may be altered and this may change (improve or decrease) its ability to interact with antibody. Changes in pH may also affect antigen conformation.
Antigens may be present in their native, cellular environment, or extracted and purified. In their natural form they may be cytoplasmic or associated with a cellular membrane, intracellular or extracellular.