Immunoglobulin G fragment antigen-binding region (IgG Fab region) (IgG Fab)

The Immunoglobulin G fragment antigen-binding region (IgG Fab) is one of two identical arms at the top portion of an IgG antibody molecule responsible for recognizing and binding specific antigens with high specificity. Each Fab consists of one constant domain and one variable domain from both the heavy chain and light chain polypeptides. The variable domains contain hypervariable loops called complementarity-determining regions (CDRs), which form the paratope—the precise site that binds epitopes on antigens. Structurally, each IgG molecule has two identical heavy chains (~50 kDa each) and two identical light chains (~25 kDa each), linked by disulfide bonds forming a flexible Y-shaped structure. Papain digestion cleaves an intact IgG into two monovalent Fabs (~50 kDa each), which retain full antigen-binding capacity but lack Fc-mediated effector functions such as complement activation or Fc receptor engagement. The hinge region between CH1 and CH2 domains provides flexibility allowing independent movement of both Fabs enabling simultaneous binding to multiple epitopes at varying distances. Functionally, the IgG Fab mediates immune defense by specifically recognizing foreign molecules like pathogens through its diverse CDR sequences generated by somatic recombination and mutation processes during B cell development. This specificity underpins adaptive immunity's ability to recognize virtually any pathogen encountered. In therapeutic contexts, isolated Fabs are used as diagnostic tools or engineered into smaller antibody fragments for improved tissue penetration while retaining specificity but lacking effector functions mediated by Fc portions.

Other names

Fragment antigen-binding (Fab)Antigen-binding fragment of IgGF(ab) fragment

Specific binding to antigens via complementarity-determining regions in variable domains of the Fab. This leads to neutralization or marking pathogens/cells for immune clearance.