Immunoglobulin Structure: Domains, Regions, and Organization

Core Idea

Immunoglobulins consist of two identical heavy chains and two identical light chains (κ or λ) forming a Y-shaped molecule. Each chain has a variable domain (VH/VL) at the tip, forming the antigen-binding site (paratope) through CDR loops, and constant domains (CH/CL) forming the Fc region. The hinge region provides flexibility, allowing antibodies to bridge antigens. Heavy chain constant regions determine isotype (IgM, IgG, IgA, IgE) and effector functions; light chains contribute to antigen binding affinity.

How It's Best Learned

Draw an antibody molecule labeling V regions, C regions, hinge, CDRs, antigen-binding site, and Fc region. Compare light chain versus heavy chain contributions to antigen binding and effector functions.

Common Misconceptions

• Heavy and light chains are equally important for antigen binding (VH contributes most of the binding affinity; VL makes critical contacts). - All antibodies have identical domain structures (domain number and location vary with isotype).

Explainer

From your study of protein structure, you know that proteins fold into domains — semi-independent structural units, each with a characteristic three-dimensional fold. Immunoglobulins are built entirely from a single type of structural module: the immunoglobulin domain, a compact β-sandwich of roughly 110 amino acids consisting of two β-sheets packed face-to-face and stabilized by a conserved disulfide bond. This domain architecture — called the immunoglobulin fold — is one of the most common protein folds in the human genome, appearing not only in antibodies but in T cell receptors, MHC molecules, and many cell adhesion molecules.

A typical antibody molecule, such as IgG, has the shape of the letter Y and is composed of four polypeptide chains: two identical heavy chains (~50 kDa each) and two identical light chains (~25 kDa each), held together by disulfide bonds and noncovalent interactions. Each light chain contains two immunoglobulin domains — one variable (VL) and one constant (CL). Each heavy chain contains four or five domains — one variable (VH) and three or four constant (CH1, CH2, CH3, and sometimes CH4). The variable domains of one heavy chain and one light chain pair together at each tip of the Y to form the antigen-binding site. Since the Y has two tips, each antibody has two identical antigen-binding sites — it is bivalent, which allows it to crosslink antigens and form immune complexes.

Within each variable domain, most of the sequence is relatively conserved and forms the structural scaffold (called framework regions). The actual antigen-contacting residues are concentrated in three short loops called complementarity-determining regions (CDRs) — CDR1, CDR2, and CDR3. The six CDR loops (three from VH and three from VL) come together in three-dimensional space to form the paratope, the surface that physically contacts the antigen's epitope. CDR3 of the heavy chain is the most variable and typically makes the most critical contacts with antigen, which is why VH generally contributes more to binding affinity than VL. The incredible diversity of antibody specificity arises primarily from variation in these CDR loops, generated through V(D)J recombination and somatic hypermutation.

The stem of the Y — formed by the paired CH2 and CH3 domains — is called the Fc region (fragment crystallizable). The Fc region does not contact antigen; instead, it determines the antibody's effector functions — what happens after antigen is bound. The Fc region is recognized by Fc receptors on phagocytes and natural killer cells, by complement component C1q, and by the neonatal Fc receptor (FcRn) that controls antibody half-life. Different heavy chain constant regions define the five antibody isotypes (IgM, IgD, IgG, IgA, IgE), each with distinct effector capabilities: IgG opsonizes and activates complement, IgE triggers mast cell degranulation, IgA protects mucosal surfaces, and IgM (a pentamer) is the first responder with powerful complement activation. Between the antigen-binding arms and the Fc stem lies the hinge region, a flexible segment rich in proline and cysteine residues that allows the two Fab arms to open and close, accommodating antigens at varying distances apart on a pathogen surface. This flexibility is essential — without it, antibodies would be rigid and unable to simultaneously engage two epitopes on the same target.