The Immune System as Chemistry: Antibodies and Antigens

The immune system is usually described in terms of cells — white blood cells patrolling, engulfing and attacking. But underneath every one of those actions is chemistry: proteins recognising the shapes of other molecules, enzymes cutting and linking, and cells releasing reactive oxygen species to kill microbes. Seen this way, immunity is a story about molecular recognition — the same principles of shape, charge and intermolecular forces that govern enzymes and drugs.

Two lines of defence

The immune system has two broad parts:

• Innate immunity: fast, general defences present from birth. It recognises broad chemical patterns common to microbes — for example, molecules in bacterial cell walls or double-stranded RNA from viruses — using receptors called pattern-recognition receptors.
• Adaptive immunity: slower at first but highly specific, and it remembers. This is where antibodies and antigens come in.

Antigens and epitopes

An antigen is any molecule that the adaptive immune system can recognise. Most are proteins or large carbohydrates on the surface of bacteria, viruses, pollen or other foreign material.

An antibody doesn’t recognise a whole antigen. It binds to a small region on it called an epitope — typically a patch of around 5 to 15 amino acids on a protein’s surface, or a few sugar units on a carbohydrate. An epitope may be a continuous stretch of the chain, or it may be made of amino acids that are far apart in the sequence but brought together when the protein folds (see protein folding). This is why denaturing a protein can destroy some epitopes: change the 3D shape and the antibody no longer recognises it.

A single large antigen, such as a viral surface protein, can carry many different epitopes, each recognised by a different antibody.

The structure of an antibody

Antibodies, also called immunoglobulins (Ig), are Y-shaped proteins made by B cells. The most common type in blood, IgG, has a molar mass of about 150,000 g mol⁻¹ and consists of four polypeptide chains:

• two identical heavy chains
• two identical light chains

The chains are held together by disulfide bridges between cysteine residues, plus non-covalent interactions — a good example of quaternary structure (see protein structure levels).

Each antibody has two functional regions:

1. Two antigen-binding sites (Fab regions), at the tips of the Y’s arms. Each site is formed by the ends of one heavy and one light chain. Within these ends are six short loops, called complementarity-determining regions (CDRs), whose amino acid sequences vary enormously between antibodies. Their side chains shape a pocket or surface that fits one specific epitope.
2. The stem (Fc region), which is constant within a class. It doesn’t bind antigen but “tells” the rest of the immune system what to do — attaching to receptors on white blood cells or activating complement proteins.

A flexible hinge between the arms and stem lets the two binding sites swing to reach epitopes at different distances apart.

How antibodies bind: non-covalent chemistry

Antibody–antigen binding is reversible and non-covalent. It uses the same forces that hold enzymes to substrates and drugs to receptors (see how drugs are designed):

• Hydrogen bonds between polar side chains
• Ionic interactions between oppositely charged groups
• Hydrophobic interactions between non-polar surfaces, releasing ordered water molecules
• Van der Waals forces across the closely fitting surfaces

Individually weak, these forces add up when the surfaces fit snugly. The strength of binding at one site is called affinity; strong antibodies have dissociation constants in the nanomolar range or lower. Because an IgG has two binding sites (and IgM, a pentamer, has ten), an antibody can bind a surface with many epitopes much more strongly overall — this combined strength is called avidity.

Binding is like an equilibrium:

antibody + antigen ⇌ antibody–antigen complex

Making millions of different antibodies

Humans can produce billions of different antibodies, but we have only around 20,000 genes. The trick is gene shuffling. In developing B cells, gene segments for the variable regions (called V, D and J segments) are cut and rejoined in different combinations, and the joins are made imprecisely, adding or removing a few bases. Heavy and light chains combine at random. The result is an enormous variety of binding sites, each made by a different B cell.

When an antigen appears, the B cells whose antibodies happen to bind it are selected and multiply. During the response, their antibody genes mutate rapidly, and the cells making tighter-binding versions are favoured — a process called affinity maturation. It’s evolution by selection, happening inside your lymph nodes over a couple of weeks.

What antibodies actually do

Antibodies rarely kill microbes on their own. Instead, they:

1. Neutralise: by covering the parts of a virus or toxin that it uses to enter cells, they physically block infection. This is how antivenoms work (see venoms and toxins).
2. Agglutinate: with two or more binding sites, antibodies cross-link microbes into clumps that are easier to clear.
3. Opsonise: coating a microbe so that phagocytes, whose receptors grab the Fc stem, can engulf it.
4. Activate complement: a cascade of about 30 blood proteins that, once triggered, builds a membrane attack complex — a ring-shaped pore that punches holes in bacterial membranes, causing them to burst.

The chemical weapons of phagocytes

When a neutrophil or macrophage engulfs a bacterium, it traps it in a membrane bubble and releases a chemical assault — the oxidative burst:

• An enzyme called NADPH oxidase transfers electrons to oxygen, making superoxide (O₂•⁻).
• This is converted to hydrogen peroxide (H₂O₂).
• In neutrophils, the enzyme myeloperoxidase combines hydrogen peroxide with chloride ions to make hypochlorous acid (HOCl) — the same active substance as in household bleach.

H₂O₂ + Cl⁻ + H⁺ → HOCl + H₂O

These reactive species oxidise bacterial proteins, lipids and DNA. It’s a reminder that reactive oxygen species aren’t only harmful by-products (see the chemistry of ageing).

Antibody classes

Class	Main features
IgG	Most abundant in blood; crosses the placenta to protect newborns
IgM	First made in a response; pentamer with 10 binding sites
IgA	In tears, saliva, mucus and breast milk; protects surfaces
IgE	Low levels; binds mast cells; central to allergies and defence against parasites
IgD	Mainly on the surface of B cells

IgE’s role in allergy is covered in histamine and the chemistry of allergies.

Vaccines: training the chemistry

A vaccine presents antigens — or instructions for making them — without causing disease, so the body produces specific antibodies and memory cells in advance. On real exposure, memory B cells respond faster and produce high-affinity antibodies (see how vaccines work).

Antibodies as tools

Because antibodies bind specific molecules so precisely, chemists and doctors use them constantly:

• Lateral flow tests (home pregnancy and COVID tests): a sample flows along a strip. Antibodies attached to coloured gold nanoparticles bind the target molecule, and a line of fixed antibodies captures the complex, making a visible line.
• ELISA tests, where an enzyme linked to an antibody produces a coloured product.
• Immunoassays for hormones and vitamins in blood (see blood chemistry).
• Monoclonal antibody drugs for cancer, autoimmune diseases and infections — identical antibodies produced from a single cell line, a technique that earned César Milstein and Georges Köhler a share of the 1984 Nobel Prize.

Key takeaways

• Antibodies bind small regions of antigens called epitopes, recognising 3D shape.
• IgG is a Y-shaped protein of four chains joined by disulfide bridges; variable CDR loops form the binding sites.
• Binding is non-covalent and reversible; affinity and avidity describe its strength.
• Gene shuffling and affinity maturation create and refine enormous antibody variety.
• Antibodies neutralise, clump, tag and activate complement; phagocytes kill with superoxide, H₂O₂ and HOCl.