How T-Cell recognizes a cell infected by a virus?
All cells chew up internal proteins all the time - it's part of the normal activity of cells to recycle those proteins into amino acids to be rebuilt into other proteins. The first step is chopping up the protein into peptides by the proteasome.
When a virus infects a cell, one key thing it needs to do to replicate is to have the cell start making viral proteins. However, some of these get chewed up, too.
Some of the peptide fragments get bound to major histocompatibility complex class 1 (MHC-1) molecules. These are then transported to the cell surface. This is the cell advertising &quot;hey, this is what I've found in the trash lately.&quot;
Sort of like antibodies, T-cells only bind certain antigens, and during development the immune system learns which antigens are self versus foreign by process of elimination: if you detect it during development, it must be yours. Everything else is fair game to attack. When a killer T-cell binds strongly to the MHC-1:peptide complex, it recognizes that this is a foreign peptide and the cell presenting it must be infected or damaged in some way, and this starts a cascade that leads to apoptosis of the infected cell.