Antigen

The antigen is a molecule that is capable of binding to the product of immune response, such as antibodies. Immunogen is a molecule that is capable of binding to the components of an immune system and thereby triggering an immune response. Therefore, an immunogen is necessarily an antigen, but antigen may not necessarily be an immunogen.

Epitopes present on the surface of the antigen allow for recognition by an antibody.

Type of antigens

Exogenous antigens

These are antigens that have entered the body externally e.g. through inhalation, ingestion, injection. Exogenous antigens will be taken up by antigen-presenting cells (APCs) and presented onto the cell surface via MHC II molecules. These will be recognized by CD4+ (T helper, Th) cells. The immune system will be activated to remove the invading pathogen expressing the foreign exogenous antigen from the body. This is the way researchers can generate specific antibodies in animals that bind to human proteins.

Endogenous antigens

These antigens arise due to viral or intracellular bacterial infection. In this case, MHC I molecules present these endogenous antigens to CD8+ (T cytotoxic, Tc) cells, which then secrete chemicals that cause the lysis of the infected cell.

Auto-antigens

An autoantigen is a protein, DNA or RNA that is recognized by the immune system to be foreign and hence becomes targeted for removal. This only occurs in patients suffering from autoimmune diseases such as Multiple Sclerosis and Systemic Lupus Erythematosus.

Viral antigens

Viral antigens are those found on invading viruses.

Tumor antigens

Tumor antigens are produced by tumor cells and trigger an immune response in the host. These antigens can be presented by either MHC I or MHC II molecules on the surface of tumor cells. Tc cells that recognize these antigens may be able to destroy tumor cells. Tumor antigens are also useful as tumor markers in identifying tumor cells.

Superantigen

This is a class of antigens that cause non-specific activation of T cells which results in high numbers of polyclonal T cells and a cytokine cascade, leading to outbreaks of rashes and fever. Some examples of superantigens are bacterial enterotoxins and exotoxins.