Antibody molecule structure
A classic immunoglobulin molecule (e.g., IgG) consists of paired polypeptide chains: two heavy (H) chains and two light (L) chains. They are held together by disulfide bonds in a flexible hinge region.
Each chain is folded into globular blocks — domains:
- Variable (V-domains) are located at the N-termini of the chains and are responsible for antigen binding.
- Constant (C-domains) form the structural framework and determine the biological properties of the molecule.
If an antibody is treated with the enzyme papain, it splits into three parts: two Fab fragments (contain the active site and bind antigen) and one Fc fragment (the "tail" region that interacts with Fc receptors of immune cells).
Immunoglobulin classes
The type of heavy chain determines which of the five isotypes (classes) an antibody belongs to. Light chains come in only two types — κ (kappa) and λ (lambda), and they are found in all classes in a 3:2 ratio.
- IgG (
γ chains) — monomers. The main antibodies of the secondary response, protecting against bacteria and viruses. They form the basis of serum immunoglobulins. - IgM (
μ chains) — giant pentamers (5 monomers held together by a J chain). Produced first upon contact with infection, excellent at activating complement, but have relatively low antigen affinity. - IgA (
α chains) — circulate as monomers in the blood, while forming dimers (secretory sIgA) on mucous membranes. They are the main factor in mucosal protection. - IgE (
ε chains) — monomers. Present in extremely low amounts in serum, but critically important for combating parasites and triggering immediate-type allergic reactions (bind to mast cells and basophils). - IgD (
δ chains) — monomers. Function mainly as B-lymphocyte membrane receptors.
How does an antibody recognize an antigen?
The secret lies in the structure of the V-domains. They contain relatively stable framework regions and three hypervariable regions (CDRs). It is the CDRs that form a unique spatial topography — the active site.
The active site is formed jointly by regions of the heavy and light chains oriented toward each other. It acts as a hydrophilic channel that precisely matches the antigen via the principle of spatial complementarity. The diversity of these regions is so vast that the body can synthesize unique antibodies to a huge number of targets.
Immunoglobulin superfamily (IgSF)
The domain structure of antibodies (two antiparallel β-sheets held together by a disulfide bond) proved so successful that nature used it to create many other molecules. The immunoglobulin superfamily includes:
- T-cell receptors (TCR);
- Major histocompatibility complex (MHC) molecules;
- Coreceptors (CD4, CD8, CD3) and adhesion molecules.
Evolutionarily, this basic structure appeared in invertebrates, long before a fully-fledged adaptive immune system was formed.
To easily remember the 5 immunoglobulin classes, use the English word GAMED (IgG, IgA, IgM, IgE, IgD).
Class and exam questions
What is the difference between Fab and Fc fragments?
The Fab fragment is responsible for specific antigen binding (contains variable domains). The Fc fragment is the constant "tail" region of the molecule, which is recognized by receptors on macrophages and neutrophils and triggers effector reactions.
Which immunoglobulins form polymers?
IgM exists as a pentamer (5 subunits), while secretory IgA exists as a dimer. In both cases, the monomers are linked by a special polypeptide J chain.
What is the difference between the action of papain and pepsin on IgG?
Papain cleaves the molecule in the hinge region into three separate parts (two Fab and one Fc). Pepsin cleaves "below" the disulfide bonds, leaving the Fab fragments linked together as a single divalent F(ab')2 fragment.
What else to study on this topic
- Spatial folding of β-sheets in C- and V-domains
- Role of the carbohydrate component (glycosylation) in antibody function
- Allotypy and idiotypy of immunoglobulins
- Secretory component (SC) and IgA transport to mucosal surfaces
- IgG and IgA subclasses: structural and functional differences
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