Antigens and Immunogens

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Immunogens and antigens are different in term of the definitions. Immunogens are the substance that can induce the immune response, basically production of B and/or T cells. It is so called immunogenic. Immunogenicity is the ability to induce the humoral and cellular immune responses.

Antigens are the substances that can be recognized by antibodies or by T cell receptors when complexed with MHC molecules. Antigens are so called antigenic.Antigenicity is the ability of antibody or T cell receptor (TCR) bind specifically.

All immunogens are also antigens. But, not all antigens are immunogens (not every chemical substance that can be bound by an antibody or TCR is, by itself, able to trigger an immune response). For example, an virus is covered by lipid structure envelope. The immune response is only directed against proteins (inside virus). Thus, protein in this case is called immunogen; while, the response is usually on directed to the few discrete amino acids within the protein.The specific location where the antibody or TCR recognizes (binds) is called epitope.

Immunogens and Immunogenicity

Few factors affect the immunogenicity:

Immunogenicity factor

1. Foreignness

  • Immune system usually differentiate self and non self . Therefore, the more foreignness of substances toward host will be more immunogenic.
  • Greater and more phylogenetic distance between two species, the greater the structural (and therefore the antigenic) disparity between them.

2. Molecular Size

  • Immunogens with molecular mass of 100,000 daltons (Da) are more immunogenic.
  • Substances with a molecular mass less than 5000–10,000 Da are poor immunogens.

3. Chemical composition and heterogeneity

  • Proteins are more immunogenic compared other macromolecules.
  • Nucleic acids and lipids are not immunogenic. Antibodies still can bind to them when they bind with proteins in form of nucleoproteins and lipoproteins. Thus, nucleic acids and lipids are antigenic but not immunogenic.

4. Degradibility

  • Humoral and cell mediated immunity normally involve the interaction of  T cells and antigens where the antigens must be processed and complexed by MHC of antigen presenting cells (APCs).
  • Large and insoluble substances are more immunogenic than smaller and soluble because they large substances are more likely to be phagocytosed and processed by APCs. Thus, they are more likely to be expressed at MHC complex.
  • The substances that are difficult to be phagocytosed are the poor immunogens.

5. Genotype of recipient

  • MHC plays a central role in determining the degree to which an animal responds to an immunogen.
  • Different genetically affects the immunogenicity of a given macromolecule in different animals.

6. Immunogens dosage and administration route

  • Different of immunogen dosage affects the immunogenicity.
  • Repeated administrations, or boosters, increase the immunogenicity by increasing the clonal proliferation of antigen-specific T cells or B cells.
  • Administration route also affect the immunogenicity.  The common administration routes are: intravenous (into vein), intradermal (into skin), subcutaneous (beneath skin), intramuscular (into muscle) and intraperitoneal (into peritoneal cavity).

7. Adjuvants

  • Adjuvants  are substances that, when mixed with an antigen and injected with it, enhance the
    immunogenicity of that antigen.
  • Adjuvants are often used to boost the immune response when an antigen has low or poor immunogenicity or when only small amounts of an antigen are available.