Fab region

Fab region is a part of an antibody that can bind to antigens. It is composed of one constant and one variable domain of each of the heavy and the light chain. These domains shape the paratope — the antigen-binding site — at the amino terminal end of the monomer. Each tip of the "Y" of an antibody contains a paratope that is specific for one particular epitope on an antigen, allowing these two structures to bind together with precision.

Structure

The Fab region of an antibody is composed of four domains: two heavy chain domains (one constant (CH1) and one variable (VH)) and two light chain domains (one constant (CL) and one variable (VL)). The variable domains from the heavy and light chains pair together to form the antigen-binding site at the tip of the Fab region. This region is known as the paratope and is highly variable, allowing millions of antibodies with different paratopes to exist. This enormous diversity of antibodies allows the immune system to recognize an equally wide variety of antigens.

Function

The Fab region is responsible for the specificity of an antibody for its matching antigen. The variable region of the Fab includes the idiotope, or the unique set of antigenic determinants of the antibody. It is this region that binds to the specific epitope of the antigen. The binding can be of high affinity, depending on the antigen, and is the first step of the immune response.

Clinical significance

In clinical practice, the Fab region is significant as it can be used to generate monoclonal antibodies. These are antibodies that are identical because they were produced by one type of immune cell and are all clones of a single parent cell. Given almost any substance, it is possible to produce monoclonal antibodies that specifically bind to that substance; they can then serve to detect or purify that substance. This has become an important tool in biochemistry, molecular biology, and medicine.

See also

• Antibody
• Antigen
• Epitope
• Paratope
• Monoclonal antibodies

Fab region Resources
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