Antigen Processing
Antigen processing, or the cytosolic pathway, is an immunological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes.
It is considered to be a stage of antigen presentation pathways. This process involves two distinct pathways for processing of antigens from an organism's own (self) proteins or intracellular pathogens (e.g. viruses), or from phagocytosed pathogens (e.g. bacteria); subsequent presentation of these antigens on class I or class II major histocompatibility complex (MHC) molecules is dependent on which pathway is used. Both MHC class I and II are required to bind antigen before they are stably expressed on a cell surface. MHC I antigen presentation typically (considering cross-presentation) involves the endogenous pathway of antigen processing, and MHC II antigen presentation involves the exogenous pathway of antigen processing. Cross-presentation involves parts of the exogenous and the endogenous pathways but ultimately involves the latter portion of the endogenous pathway (e.g. proteolysis of antigens for binding to MHC I molecules).
While the joint distinction between the two pathways is useful, there are instances where extracellular-derived peptides are presented in the context of MHC class I and cytosolic peptides are presented in the context of MHC class II (this often happens in dendritic cells).
The antigen is recognized by two distinct sets of highly variable receptor molecules.
1. The antigen receptors on B cells
(The immunoglobulins that serve as
)2. The antigen-specific receptors of T cells.
antigen-specific receptors of T cells
T cells recognize only antigens that are displayed on cell surfaces.
These antigens may derive from pathogens that replicate within cells, such as viruses or intracellular bacteria, or from pathogens or their products that cells internalize by endocytosis from the extracellular fluid. T cells can detect the presence of intracellular pathogens because infected cells display on their surface peptide fragments derived from the pathogens' proteins.
These foreign peptides are delivered to the cell surface by specialized host-cell glycoproteins, the MHC molecules. The MHC glycoproteins are encoded in a large cluster of genes that were first identified by their potent effects on the immune response to transplanted tissues. For that reason, the gene complex was termed the major histocompatibility complex (MHC). We now know that within this region of the genome, in addition to those genes encoding the MHC molecules themselves, are many genes whose products are involved in the production of the MHC:peptide complexes.
Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen.
It is specific for the pathogen and confers protective immunity to reinfection with that pathogen.
Adaptive immunity can specifically recognize and destroy the pathogen because lymphocytes carry specialized cellular receptors and produce specific antibodies.
A protein that is produced in response to a particular pathogen is called the antibody, and the substance that induces the production of antibodies is called the antigen.
Paul Ehrlich coined the term antibody (in German Antikörper) in his side-chain theory at the end of the 19th century.[10] In 1899, Ladislas Deutsch (Laszlo Detre) (1874–1939) named the hypothetical substances halfway between bacterial constituents and antibodies "substances immunogenes ou antigenes" (antigenic or immunogenic substances). He originally believed those substances to be precursors of antibodies, just as zymogen is a precursor of an enzyme. But, by 1903, he understood that an antigen induces the production of immune bodies (antibodies) and wrote that the word antigen is a contraction of antisomatogen (Immunkörperbildner). The Oxford English Dictionary indicates that the logical construction should be "anti(body)-gen".[11]