In their natural form they may be cytoplasmic or associated with a cellular membrane, intracellular or extracellular. If a gene product of interest is present in extremely low concentrations, one may choose to use known nucleotide sequence information to derive a corresponding peptide for generating sequence-specific antibodies. In some instances, peptides antigens have advantages over whole proteins antigens in that the antibodies generated may be targeted to unique sequence regions. This is especially useful when investigating proteins which belong to families of high sequence homology. They are produced in response to the invasion of foreign molecules in the body.
Antibodies exist as one or more copies of a Y-shaped unit, composed of four polypeptide chains.
Heavy chains of IgG, IgM, IgA, IgD, and IgE, are known as gamma, mu, alpha, delta, and epsilon, respectively. The bonding between antigens and antibodies is dependent on hydrogen bonds, hydrophobic bonds, electrostatic forces, and van der Waals forces. These are all bonds of a weak, non-covalent nature, yet some of the associations between antigen and antibody can be quite strong. Like antibodies, antigens can be multivalent, either through multiple copies of the same epitope, or through the presence of multiple epitopes which are recognized by multiple antibodies. Interactions involving multivalency can produce more stabilized complexes, however multivalency can also result in steric difficulties, thus reducing the possibility for binding.