Antibodies

Antibodies (also called immunoglobulins, Ig) are Y-shaped proteins secreted by plasma cells that bind antigens with exquisite specificity. They are the effectors of the humoral immune response: circulating in blood and lymph, tagging pathogens for destruction and neutralising their toxins. OCR specification 4.1.1 (g) requires you to know the structure of an antibody, and the roles of agglutination, opsonisation and neutralisation in destroying pathogens.

Key Definitions:

• Antibody (immunoglobulin) — a glycoprotein produced by plasma cells that binds specifically to an antigen.
• Antigen-binding site — the region of an antibody that binds antigen; determined by the variable region.
• Epitope — the specific small region of an antigen that an antibody recognises.
• Disulfide bridge — an S–S covalent bond between two cysteine residues.

---

Antibody Structure

Antibodies are Y-shaped glycoproteins with a remarkable combination of variety at one end and constancy at the other.

Chains

Each antibody is composed of four polypeptide chains:

• Two identical heavy chains (~50 kDa each).
• Two identical light chains (~25 kDa each).

The chains are held together by disulfide bridges (covalent S–S bonds between cysteine residues) and non-covalent interactions.

Regions

Each chain is divided into:

• Variable regions at the tips of the Y, forming the antigen-binding sites. The amino acid sequence varies between antibodies, giving each antibody its unique specificity. There are two antigen-binding sites per antibody (one on each arm of the Y).
• Constant regions forming the "stem" (Fc region) of the Y. The amino acid sequence is the same within a class of antibody, and determines the antibody's effector function (which Fc receptors it binds).

Hinge Region

Between the arms and the stem is a flexible hinge region that allows the two arms to move independently, enabling the antibody to bind two antigens at slightly different distances apart.

flowchart TD
    A[Antigen-binding site] --> B[Variable region, heavy chain]
    A --> C[Variable region, light chain]
    B --> D[Hinge region]
    C --> D
    D --> E[Constant region, heavy chains]
    E --> F[Fc region: binds Fc receptors, complement]

Key Features Summary

Feature	Location	Role
Two heavy chains	Whole Y	Structural backbone
Two light chains	Arms	Structural, antigen binding
Variable region	Tips of arms	Bind epitope; unique to each antibody
Constant region	Stem (Fc)	Effector function; binds receptors
Hinge	Middle	Flexibility
Disulfide bridges	Between and within chains	Hold structure together
Glycosylation	Fc region	Affects binding to Fc receptors

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.