= Fragment antigen-binding
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= Fab fragment
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A <Fab fragment> contains one <antibody binding site>, allowing comparison with the two-site intact <IgG> in the <antibody surface plasmon resonance experiment>.
A <Fab fragment> combines one complete <immunoglobulin light chain> with one <immunoglobulin heavy chain> $V_H,C_{H1}$ portion, including <antibody constant regions>.<Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others/The structure of a typical antibody molecule>[Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others; §3-3: complete light chain paired with heavy-chain VH and CH1 in Fab]{ref}
The single <paratope> makes a <Fab fragment> useful for separating <antibody affinity> from two-arm <avidity>. If an intact <IgG> attaches strongly to a surface but its matched <Fab fragments> detach more readily, loss of the second arm is a candidate explanation even if the individual <paratope> is unchanged. Comparing the formats asks a structural question; it does not by itself establish which binding mechanism dominates.
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