Practice question

Antibody specificity arises exquisite three-dimensional complementarity between paratope heavy light chain variable domains epitope antigen surface determining binding free energy selectivity. Variable domains each contribute three complementarity-determining regions CDR1 CDR2 CDR3 loops supported conserved framework beta sheet Ig fold creating pocket groove protruding loop accommodating antigen features charged arginine aspartate hydrophobic tryptophan phenylalanine patches hydrogen bond donors carbohydrate. Binding stabilized hydrogen bonds CDR backbone carbonyl antigen side chain salt bridges arginine aspartate van der Waals contacts burying 600-900 square angstrom entropic gain displacing ordered water. Equilibrium dissociation constants after affinity maturation involving activation-induced cytidine deaminase mediated somatic hypermutation point mutations rate 10^-3 per base division selection improved binding germinal center dark zone light zone cycle follicular dendritic cell antigen presentation T follicular helper CD40 ligand signals reach picomolar 10^-11 M. Specific antibody discriminates closely related antigens differing single amino acid HER1 vs HER2 despite 40 percent homology or dopamine vs norepinephrine preventing cross-reactivity contributing safety therapeutic monoclonal targeting CD20 without depleting CD22 or HER2 without affecting EGFR. Specificity underpins diagnostic accuracy toxin neutralization blocking receptor interaction therapeutic targeting.