Fab fragment

Fab fragment refers to a region on an antibody that binds to antigens. It is one of the two arms of the Y-shaped antibody structure, with the other being the Fc (fragment crystallizable) region. The Fab ("fragment, antigen-binding") portion is crucial for the specificity of antibodies for their particular antigens. Each antibody contains two Fab fragments, which allow it to bind to specific antigens, thereby facilitating the immune response.

Structure

The Fab fragment consists of one constant and one variable domain from each of the heavy and the light chains of the antibody. The variable domains, which are part of the V region of the antibody, are responsible for antigen recognition. This specificity is due to the unique sequence of amino acids in the variable regions, which forms a three-dimensional structure that can bind to a complementary structure on the antigen.

Function

The primary function of the Fab fragment is to bind to antigens. This binding is the first step in the immune response to invading pathogens or foreign substances. Once an antigen is bound, the antibody can neutralize the pathogen directly or mark it for destruction by other components of the immune system, such as phagocytes or the complement system.

Production and Use

Fab fragments can be generated by enzymatic digestion of antibodies with enzymes such as papain, which cleaves the antibody molecule just above the disulfide bond that links the two heavy chains. This process yields two Fab fragments and one Fc fragment. Fab fragments are used in various research and clinical applications, including as therapeutic agents in diseases where modulation of the immune response is desired. They offer the advantage of antigen specificity without the effector functions mediated by the Fc region, reducing the risk of certain side effects.

Applications in Medicine

In medicine, Fab fragments are used in the treatment of conditions such as acute toxicity caused by drug overdose. For example, digoxin-specific Fab fragments are used to treat overdoses of digoxin, a cardiac glycoside. The Fab fragments bind to the drug molecules, forming complexes that are then removed from circulation by the kidneys.

Advantages and Limitations

The use of Fab fragments offers several advantages, including reduced immunogenicity compared to full-length antibodies and the ability to penetrate tissues more effectively due to their smaller size. However, their smaller size also means they are cleared from the bloodstream more quickly, which can limit their therapeutic efficacy in some applications.