Fragment antigen-binding region

The fragment antigen-binding region (Fab) is a crucial part 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 Fab region is responsible for the specific recognition and binding of antigens, which is essential for the immune response.

Structure

Structure of a Fab fragment

The Fab region consists of two identical light chains and two identical heavy chains. Each chain is composed of a variable domain (V) and a constant domain (C). The variable domains of the heavy and light chains form the antigen-binding site, which is highly specific to the antigen it binds. The constant domains provide structural support and stability to the Fab region.

Diagram showing the separation of Fab and Fc regions

The variable region of the Fab is where the complementarity-determining regions (CDRs) are located. These CDRs are hypervariable loops that determine the specificity and affinity of the antibody for its antigen.

Function

Diagram of an antibody showing the Fab and Fc regions

The primary function of the Fab region is to bind to antigens. This binding is highly specific, allowing the immune system to target and neutralize pathogens effectively. The Fab region can recognize a wide variety of antigens due to the diversity generated during somatic recombination and somatic hypermutation in B cells.

Once the Fab region binds to an antigen, it can neutralize the pathogen directly or mark it for destruction by other components of the immune system, such as phagocytes or the complement system.

Applications

Fab fragments are used in various therapeutic and diagnostic applications. They can be engineered to improve their binding affinity and specificity for certain antigens. Fab fragments are smaller than whole antibodies, which allows them to penetrate tissues more effectively and be cleared from the body more rapidly.

In clinical settings, Fab fragments are used in the treatment of certain conditions, such as snakebites, where they can neutralize venom. They are also used in immunoassays and as biological markers in research.

Comparison with Fc Region

Diagram showing the Fab and Fc regions of an antibody

The Fab region is distinct from the fragment crystallizable region (Fc) of an antibody. While the Fab region is involved in antigen binding, the Fc region mediates interactions with cell surface receptors and the complement system. The Fc region is responsible for the effector functions of antibodies, such as antibody-dependent cellular cytotoxicity (ADCC) and complement activation.

Related pages

• Antibody
• Antigen
• Immunoglobulin
• Complement system
• B cell