Practice question

Monoclonal antibody originates from unique genetic event where one developing B cell successfully completed VDJ recombination on heavy chain locus chromosome 14 mediated by RAG1/2 recognition of recombination signal sequences, non-homologous end joining with TdT adding N nucleotides creating unique complementarity determining region three loop that dominates antigen contact, plus VJ recombination on light chain locus producing paired variable domains. All progeny via mitosis carry identical nucleotide sequence encoding paratope whose three-dimensional shape, electrostatic surface, and hydrophobic pockets complement single epitope topology defined by side chain arrangement and glycan. Binding affinity Kd remains uniform across preparation because combining site does not vary, allowing quantitative analysis via Scatchard plot and predictable off-rate. Even bivalent IgG displays same intrinsic affinity per Fab arm, avidity only increases with multivalent antigen. Such uniformity reduces nonspecific background in immunohistochemistry where inconsistent binding creates noise, enables precise epitope mapping using alanine scanning mutagenesis, and ensures reproducible pharmacokinetics with linear clearance. Uniform binding to single epitope with constant affinity distinguishes monoclonal from heterogeneous polyclonal mixture containing varying avidities and cross-reactivities that compromise quantitation.