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.
Exogenous Antigens
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
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.
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