Final answer:
T cell epitopes are recognized due to the specificity of TCRs for particular amino acid sequences and shapes of antigen-binding sites. The TCR diversity, generated by mutation and recombination, allows the immune system to recognize multiple pathogens. Activation and clonal expansion of T cells occur only for those cells with receptors specific to the antigenic epitope presented.
Step-by-step explanation:
T cell epitopes recognize sequential antigenic epitopes because of the specificity of the T cell receptor (TCR) to the amino acid sequence and the three-dimensional shape of the antigen-binding site. Antigen-presenting cells (APCs) only present the most antigenic or immunodominant epitopes. These are then displayed in association with MHC II molecules. The TCR diversity is created through gene mutation and recombination in T cell precursors, allowing the immune system to recognize a vast array of pathogens. The clonal selection mechanism ensures that only T cells with receptors specific to the presented epitope are activated, leading to clonal expansion of the appropriate T cell population.
Cross-reactivity may occur when antibodies recognize similar epitopes on different antigens due to the small surface area defining an epitope. This allows the adaptive immune system to effectively generate immune responses to various pathogens encountered throughout an individual's lifetime.