The Y shape, the four chains, the V and C regions, the hinge, the disulfide bridges and the Fc stem are the labels OCR examiners reward. Drawing this clearly is worth several marks in any 6-mark "describe the structure of an antibody" question.

Key Features Summary

Classes of Antibody

Five classes of antibody exist, distinguished by their heavy chain constant region:

• IgG — most abundant (~75% of serum antibody); main antibody in secondary responses; crosses placenta.
• IgM — pentameric (five Y-units linked); first antibody made in primary response; powerful agglutinator.
• IgA — dimeric in secretions (saliva, milk, mucus); protects mucous membranes.
• IgE — binds mast cells; responsible for allergies and anti-parasite responses.
• IgD — B cell receptor; function in secretions less clear.

For OCR A-Level, you need to know the general Y-shaped structure (IgG is the usual model).

How Antibodies Destroy Pathogens

Antibodies themselves do not kill pathogens directly. Instead, they mark pathogens for destruction by other parts of the immune system. There are three main effector mechanisms you must know:

1. Neutralisation

Antibodies bind to toxins, viruses or bacterial surface proteins and block their ability to damage host cells.

• Viral surface proteins (e.g., HIV gp120, influenza HA) can be "neutralised" by antibodies that block them from attaching to host cell receptors.
• Bacterial toxins (e.g., tetanospasmin) can be neutralised by binding antibodies so they can no longer interact with host proteins.

Neutralisation is especially important for viruses — a good vaccine induces neutralising antibodies.

2. Agglutination

Each antibody has two antigen-binding sites, and antibodies can therefore cross-link multiple pathogens by binding the same antigen on different cells. The result is a large clump of pathogens that:

• Cannot spread through the body as easily.
• Is more readily engulfed by a single phagocyte.

IgM, with its pentameric structure (10 antigen-binding sites), is the most powerful agglutinator and is why IgM dominates the early primary response.

Blood group antigens also cause agglutination — the basis of ABO blood grouping and transfusion compatibility.

3. Opsonisation

Antibodies bound to a pathogen act as opsonins, marking it for phagocytosis. The Fc region of the antibody is recognised by Fc receptors on macrophages and neutrophils — chiefly FcγRI, FcγRIIA and FcγRIIIA for IgG. The receptor engagement triggers actin-mediated engulfment, and opsonised pathogens are phagocytosed many times faster than non-opsonised ones. Opsonisation is the basis of intravenous immunoglobulin (IVIG) therapy in immunodeficiencies and of monoclonal-antibody therapies for HER2-positive breast cancer (trastuzumab opsonises HER2-expressing tumour cells for natural-killer-cell killing, antibody-dependent cellular cytotoxicity).

4. Complement Activation

Antibody–antigen complexes (specifically IgG and IgM) activate the classical complement pathway: C1q binds the Fc region of clustered antibodies, triggering the proteolytic cascade C1 → C4 → C2 → C3 → C5–C9. The cascade produces:

• C3b — a powerful opsonin that coats the pathogen and binds complement receptors on phagocytes.
• C3a, C5a — anaphylatoxins that recruit and activate inflammatory cells.
• Membrane attack complex (MAC) — the C5b-C9 assembly that polymerises into the pathogen membrane, forming a transmembrane pore that lyses the pathogen osmotically.

Gram-negative bacteria are particularly vulnerable to MAC-mediated lysis; gram-positive bacteria with thick peptidoglycan walls are largely resistant. Antibody-mediated complement activation is therefore a major mechanism by which IgG and IgM destroy susceptible bacteria — a quantitative effect easily seen in serum bactericidal assays.

flowchart LR
    A[Antibody binds antigen] --> B{Effector action?}
    B --> C[Neutralisation: blocks toxin/virus]
    B --> D[Agglutination: clumps pathogens]
    B --> E[Opsonisation: Fc binds phagocyte receptor]
    B --> F[Complement activation: MAC lyses bacteria]
    C --> G[Pathogen inactivated]
    D --> H[Pathogen engulfed]
    E --> H
    F --> I[Pathogen lysed]

Specificity: Why Each Antibody Only Binds One Antigen

An antibody's antigen-binding site is shaped by the variable regions of the heavy and light chains. Only an antigen of complementary shape and chemistry can fit, forming non-covalent bonds (hydrogen bonds, ionic bonds, van der Waals forces) with residues lining the binding pocket. This is an example of a lock-and-key fit analogous to enzyme–substrate specificity.

The variable region sequence is generated by V(D)J recombination during B cell development: random shuffling of gene segments in the bone marrow produces an estimated 10¹¹ different possible antibody specificities.

Monoclonal Antibodies — Production and Applications

Polyclonal antibodies are the natural mix of antibodies produced by many B-cell clones during an immune response, each clone binding a different epitope of the antigen. Monoclonal antibodies (mAbs) are produced by a single B-cell clone, bind a single epitope and are chemically homogeneous — making them ideal reagents for both diagnostics and targeted therapy. The technology was invented by Georges Köhler and César Milstein at the Cambridge MRC Laboratory of Molecular Biology in 1975 (Nobel 1984).

Hybridoma production