Fab Fragment
Definition
The antigen-binding fragment of an antibody, consisting of one complete light chain paired with the variable and first constant domain of a heavy chain. Each IgG molecule has two Fab arms and therefore two identical antigen-binding sites.
- Composition
- One light chain plus the variable and first constant domain of a heavy chain
- Function
- Antigen binding
- Count per IgG
- Two identical Fab arms per antibody molecule
- Companion fragment
- Fc fragment, the stem region, which is not involved in antigen binding
Common questions
Why does the Fab fragment matter in forensic serology or immunoassay work?+
Antibody-based tests used to identify body fluids or species rely on the Fab region's specificity for its target antigen, so understanding Fab structure explains why some antibody reagents cross-react with related antigens and others do not.
How does a Fab fragment differ from a whole antibody in practical use?+
Because a Fab fragment lacks the Fc stem, it cannot trigger the immune effector functions (like complement fixation) that depend on Fc receptors, which is why isolated Fab fragments are sometimes used in assays to avoid unwanted secondary binding.
Related terms
- Affinity
- The binding strength of a single antigen-binding site for one epitope, expressed as the equilibrium dissociation constant (Kd). A lower Kd indicates...
- Complementarity-Determining Regions (CDRs)
- Six hypervariable loops, three in the heavy-chain variable domain and three in the light-chain variable domain, that together form the antigen-binding site....
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Fc Region
- The crystallisable fragment of an antibody, formed by the paired constant domains of the two heavy chains. The Fc region mediates effector...
- Monoclonal Antibody (mAb)
- An antibody produced by a single hybridoma clone, so that every molecule in the preparation is identical, targets the same epitope, and...