Final answer:
T cell receptors (TCRs) recognize antigens that are processed and presented with MHC molecules on the surface of antigen-presenting cells. This interaction is specific and leads to immune responses such as the activation and proliferation of T cells specific to the pathogen, known as clonal expansion.
Step-by-step explanation:
TCR and Antigen Recognition
T cell receptors (TCRs) recognize antigens (short peptides) generated by degradation of pathogen's proteins because TCRs are involved in the immune response by identifying processed foreign epitopes presented alongside self MHC molecules. The process starts with a pathogen being internalized by an antigen-presenting cell through phagocytosis. Inside the cell, the pathogen is degraded into peptide fragments which are then presented on the cell's surface, associated with MHC molecules. This MHC-antigen complex is what TCRs on T cells specifically recognize. In the case of cytotoxic T cells, when they interact with an MHC I-epitope complex on an infected cell, they produce enzymes like granzymes and perforins to kill the infected cell.
Antigen Processing and Presentation
The antigen processing involves breaking down the antigen into smaller peptides, which are displayed on the surface of antigen-presenting cells. This allows TCRs to identify and bind to these antigenic peptides via MHC, initiating an immune response. The specificity of the TCR for a particular antigen-MHC complex ensures that the immune system can distinguish between self and non-self, leading to the activation of T cells that are specific to the pathogen encountered. This process, known as clonal expansion, results in a proliferation of T cells targeted against the specific pathogen.