When an intracellular pathogen, such as a virus, infects a cell, MHC class 1 molecules offer the
viral-derived peptides for T cells to recognize. Extracellular pathogens are broken down and
internalized by specialized cells called antigen-presenting cells The MHC class 1 molecules then
present these pathogens' peptide snippets for identification by T lymphocytes. Antigen processing
is the method used to break down infections and their byproducts into peptide fragments. Inside
the cell, these peptide fragments merge with MHC molecules. Thusly created, the MHC peptide
complex moves to the cell surface where it exposes a peptide fragment to t-cells.
In a cell's cytoplasm, antigen processing takes place. A cell is referred to as a target cell if a virus
has infected it. When a virus infects a cell, it uses the cell's cellular machinery to create viral
proteins, which take place in the target cell's cytoplasm. A big barrel-shaped structure known as
a proteasome absorbs and digests these proteins. Twelve to fifteen protein subunits make up
proteasomes, which are capable of proteolysis. Thus, they are able to break down cytosolic
proteins into peptide fragments that range in length from 8 to 15 amino acids. Similar to this, the
proteasome breaks down viral proteins into peptide fragments. These peptides are subsequently
transferred into the rough endoplasmic reticulum.
Through a pore created by TAG, transportation occurs (Transporters associated with Antigen
Processing). Two subunits make up TAP (TAP 1 & TAP 2) The endoplasmic reticulum is where
the MHC class 1 molecule's alpha and beta chains are created and transported. These chains are
put together, and a collection of chaperone proteins moves this molecule close to the tab site so
that when the peptides enter the endoplasmic reticulum, they bind to the MHC class 1 molecule in
the peptide binding groove. After that, the MHC class 1 peptide complex that has been created is
released from the chaperones and travels to the cell surface via the Golgi apparatus. The T-cells
can recognize it after it integrates into the membrane at the cell surface. T-cell receptors on t-cells
allow them to recognize the peptides that MHC molecules present for the purpose of recognition.
Since this pathway involves MHC class 1 molecules, it is also referred to as the MHC 1 antigen
presentation pathway. It also has the co-receptor cd8, which recognizes the Alpha 3 domain of
MHC class 1 molecule. As a result, t-cells interact with both peptide and MHC molecule on the
surface of the target cell.