Final answer:
Diversity in MHC and TCR specificity arises from structural differences and distinct genetic mechanisms; MHC class I and II have different cellular distributions and activate different T cell subsets, while TCR specificity is generated through genetic recombination in the thymus.
Step-by-step explanation:
The source of diversity in specificity between Major Histocompatibility Complex (MHC) and T-cell Receptors (TCR) lies in their structural differences and mechanisms of action. MHC class I and II molecules differ in their cellular distribution and the T-cell subsets they activate. MHC I molecules are found on most nucleated cells and present antigens to CD8+ cytotoxic T cells, while MHC II molecules are found on professional antigen-presenting cells (APCs) and present antigens to CD4+ helper T cells.
The specificity of TCRs, on the other hand, is generated through a process of genetic recombination that occurs in the thymus during T cell development, resulting in millions of T cells each with a unique TCR capable of recognizing a specific antigenic peptide-MHC complex.