rules for CDR3 length pairing

The binding of TCR CDR3 or BCR CDR3 to antigenic epitopes is essential for adaptive immune responses. Analyzing the pairing rules of β chain and α chain (or heavy chain and light chain) CDR3 lengths during germline V(D)J recombination and selection processes from a large number of single T or B cells has posed a significant challenge to immunologists. In this study, we leveraged scVDJ-seq data from public databases to investigate the length distribution range, pairing rules, and pairing differences of the two polypeptide chains in CDR3 within TCRs or BCRs at the individual T cell or B cell level. Our primary finding is that CDR3 pairing adheres to a primary rule where the β chain (or heavy chain) is generally longer than the α chain (or light chain). Notably, the difference in CDR3 length between the BCR heavy and light chains is much greater than that between the TCR β and α chains. This study offers novel insights into the structural and molecular mechanisms underlying T or B cell responses to antigens.