Any thing that makes the immune system produce a response is referred as “antigen”.
Antigens may be living foreign bodies such as viruses, bacteria, and fungi that cause disease and infection or non living agents such as dust, chemicals, pollen grains, or food proteins that can cause allergic reactions in the host.
All antigens may not be foreign bodies; they may be produced in the body itself. For example, cancer cell antigens that are produced inside by the body.
The intensity or strength of immunological response that an antigen can generate is referred as “immunogenicity”. The nature and strength, of immune response is not identical for all the antigens.
Generally proteins and polysaccharides are potent immunogens. Nucleic acids and synthetic polypeptides are also immunogenic but not strong immunogens.
Certain substances such as nylon, plastic, or Teflon that rarely display antigenic properties are referred as “tolerogens”. A tolerogen can become an immunogen if there is any alteration in its molecular form.
Hence immunogenicity of an antigen depends on certain physico chemical properties of the antigen molecules.