Endogenous vs Exogenous Antigens
Any molecule or substance that reacts to a product of a specific immune response and stimulates antibody generation is considered as an antigen. The antibody generation by an antigen is called antigenicity of that particular molecule. Antigens can be either a protein or a polysaccharide. The antigen uptake, antigen processing, and antigen presentation are mediated by antigen presenting cells (APCs), such as dendritic cells. Depending on the immune activity, antigens can be classified as immunogens, tolerogens, or allergens. Antigens may also be classified according to their origin as exogenous or endogenous.
The vast majority of antigens are exogenous antigens. They are introduced into the body from outside via various infections agents such as bacteria, viruses, fungi, protozoa, helminthes etc., or environmental substances such as dust mites, foodstuff, pollen etc. by inhalation, ingestion, or injection. APCs can actively intake exogenous antigens by endocytosis or phagocytosis and process into fragments in order to initiate the antigen- processing pathways. After initiating the pathway, the fragments are presented on the membrane together with MHC class II molecules and are recognized by TH cells.
Endogenous antigens are generated within the cells due to normal cell metabolisms or due to an intracellular bacterial or a viral infection. They can be found within the cytoplasm of APCs as self- cell proteins that are covalently linked to ubiquitin; hence they do not require active phagocytosis. When antigen- processing pathways are initiated, endogenous antigens are degraded and generated peptides by proteases. These peptides are then presented by making a complex with MHC class I molecules on the cell surface. Followed by the recognition, Tcyt cells begin to secrete compounds that cause lysis or apoptosis of infected cells. Some examples for endogenous antigens include self-antigens, tumor antigens, alloantigens, and some viral antigens where the viruses are able to integrate proviral DNA into the host’s genome.
What is the difference between Endogenous and Exogenous Antigens?
• Exogenous antigens are foreign compounds entered the body from outside, whereas exogenous antigens are the compounds that have been generated within the body.
• Exogenous antigens are actively taken up into antigen presenting cells, whereas endogenous antigens are already present within the cytoplasm of antigen presenting cells.
• Active phagocytosis is required to take up exogenous antigens, unlike endogenous antigens.
• An endogenous antigen can be a tumor- or virus-derived product. In contrast, exogenous antigen can be a product of viruses or bacterial cells that are processed by antigen presenting cells.
• In antigen processing pathways, endogenous antigens are recognized in association of MHC class I, whereas exogenous antigens are recognized in association of MHC class II.
• Endogenous antigens on MHC class I molecules are recognized by Tcyt cells. In contrast, exogenous antigens presented on MHC class II molecules are recognized by TH cells.