Fab Introduction

2020-11-11 Hits(1096)

Antigen binding fragment (Fab) is a common recombinant antibody fragment with a bigger size, a better stability and affinity.


Fab Structure:


The generic antibody (also called an immunoglobulin or Ig) is a vaguely “Y”-shaped structure composed of four disulfide-linked proteins – two smaller (25 kD) light chains and two larger (50 kD) heavy chains. The N-termini of both chains have genetically variable domains that are the source of the tremendous antigen-binding diversity of the total antibody repertoire. The C-termini of each chain are genetically homogeneous for any given Ig, and are designated as constant domains (CL or CH). The two “arms” of the Ig are each composed of heavy and light chains, and are called the Fab or antigen-binding fragment of the molecule; the “body” of the Ig is composed of paired heavy chain constant regions, and is called the Fc portion of the molecule.

Fab fragments can be obtained by protease digestion of the full-length antibody. The papain can be used to cleave an immunoglobulin monomer into two Fab fragments and an Fc fragment. Fab fragments show higher affinities or stability than scFv.

A Fab fragment is a domain of an antibody that binds to antigens. It is composed of one constant and one variable domain of each of the heavy and the light chain. The composition of Fab includes light chain variable domain(VL), light chain constant region(CL), heavy chain variable domain(VH), and heavy chain constant region 1(CH1).


Fab Construction:


Three different methods are used for generating the Fab fragments:

*Screening from phage display library: Like scFv, Fab library is a common method to generate a Fab fragment.

*Expressing a Fab fragment using recombinant technology: Recombinant Fab fragments can be obtained using mammalian expression systems, such as CHO or HEK293 cells.

*Enzymatic cleavage of the parent antibody: A two step protocol is generally employed for producing Fab fragments. Enzymatic cleavage of the whole antibodies forms F(ab’)2 fragments, followed by reduction of those fragments to yield Fab fragments.


Fab Application:


Fab fragments can eliminate non-specific binding to cellular Fc receptors and serum factors. Like scFv, Fab fragments also have good tumor penetration, rapid blood clearance, and low retention times in non-target tissue.