Antibody fragment-lysozyme complex. Molecular model of a complex formed between antibody fragment chains (green and red) and a lysozyme molecule (blue). The antibody fragments are Fab (fragment antigen-binding) protein chains, here a light chain and a heavy chain. The lysozyme is the antigen to which this antibody binds. Lysozymes are enzymes found in a wide range of biological fluids such as tears, saliva and milk. This lysozyme is from chicken egg white, and this monoclonal anti-lysozyme antibody fragment is known as Fab D1.3.