OVERVIEW OF THE IMMUNE SYSTEM
Section I. ANTIGENS
a. An antigen is traditionally defined as any substance that will cause
production of antibodies and which reacts specifically with those antibodies. This term,
however, is incomplete because it emphasizes the production of immunoglobulins.
Therefore, the term immunogen was introduced to include biological processes
involving proliferation of lymphocytes and synthesis of specific substances or
recognition molecules which can specifically combine with the inducing antigen. In
physiochemical terms, antigens are macromolecules that possess a high degree of
internal chemical complexity. They are soluble or easily solubilized by phagocytic cells
of the animal and are foreign to the animal.
b. A hapten is defined as a small molecule which by itself cannot stimulate
antibody synthesis but will combine with the antibody once formed. When the hapten is
conjugated to a protein molecule called a carrier, it can elicit an immune response.
Immunogenicity may be defined as that property of a substance (immunogen)
that endows it with the capacity to provoke a specific immune response. Antigenicity,
on the other hand, is the property of a substance (antigen) that allows it to react with the
products of the specific immune response. Substances that are immunogenic are
always antigenic, but antigens are not necessarily immunogenic.
a. Antigen Factors.
(1) Molecular weight. As a general rule, for a molecule to be immunogenic
it should have a molecular weight of 10,000 or more. The greater the molecular weight
of a substance, the more likely it is to function as an antigen.
(2) Molecular complexity. Large molecular size alone is not enough to
confer antigenicity on a substance. A molecule must possess a certain degree of
chemical complexity; generally, immunogenicity increases with structural complexity.
(3) Solubility. Molecules that are insoluble in body fluids and cannot be
converted to a soluble form by tissue enzymes are poor antigens.
(4) Foreignness. A substance that the body does not recognize as
belonging to or being a part of itself.