> ## Documentation Index
> Fetch the complete documentation index at: https://docs.adaptyvbio.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Fab

> Antibody fragment containing one antigen-binding site

Fab fragments contain one complete antigen-binding site with both heavy and light chain variable and constant domains.

<CardGroup cols={2}>
  <Card title="Size" icon="ruler">
    \~50 kDa (1/3 of IgG)
  </Card>

  <Card title="Valency" icon="link">
    Monovalent (1 binding site)
  </Card>

  <Card title="Stability" icon="shield">
    More stable than scFv
  </Card>

  <Card title="Production" icon="flask">
    Bacterial, yeast, mammalian
  </Card>
</CardGroup>

## Structure

<Tip>
  Fab maintains the native antibody binding interface while being significantly smaller than full IgG.
</Tip>

* **VH + CH1** domains (heavy chain)
* **VL + CL** domains (light chain)
* Connected by disulfide bonds

## Key Applications

* **Therapeutics** - Reduced immunogenicity
* **Diagnostics** - Fast tissue penetration
* **Crystallography** - Easier to crystallize
* **Research** - Structural studies