paratope

A paratope is the part of an antibody or B-cell receptor that directly binds to an antigen. It is formed by residues in the variable regions of the heavy and light chains and constitutes the antibody’s antigen-binding site. The paratope recognizes specific epitopes on antigens and is the functional counterpart to the epitope.

In most antibodies, the paratope is largely composed of the complementarity-determining regions (CDRs), particularly CDR H3, together with surrounding framework residues that contact the antigen. The arrangement of these residues creates a surface with the appropriate shape, charge, and hydrophobicity to engage a corresponding epitope.

Binding properties of the paratope depend on structural complementarity and chemistry. Affinity and specificity arise from how well the paratope and epitope fit, including hydrogen bonds, van der Waals interactions, and electrostatic forces. The paratope is shaped during B cell development through V(D)J recombination and somatic hypermutation, producing a diverse repertoire. Some paratopes are highly specific for a single epitope, while others can be more cross-reactive or polyreactive.

Understanding paratopes is important for antibody engineering, epitope mapping, and therapeutic design. Structural and biochemical approaches, including X-ray crystallography, cryo-electron microscopy, NMR spectroscopy, and computational modeling, are used to define paratope residues and to elucidate how they confer binding to particular antigens.