11.2 Antibodies and Vaccination

1. The Structure of Antibodies and How It Relates to Their Functions

What Is an Antibody?

An antibody is a special protein made by your immune system to fight off harmful invaders like bacteria and viruses. Antibodies are also called immunoglobulins (Ig) — these two words mean exactly the same thing.

Antibodies are described as globular glycoproteins:

• Globular means they are roughly spherical (ball-like) in shape. This happens because the side-chains of the amino acids (called R groups) that prefer water point outwards.
• Glycoprotein means the protein has short chains of carbohydrate (sugar) molecules attached to it.

The Shape of an Antibody

An antibody is shaped like the letter Y. It is made of four polypeptide chains (chains of amino acids joined together):

• Two heavy chains — these are the long chains that form the stem and the lower part of the arms of the Y
• Two light chains — these are shorter chains that run alongside the tops of the heavy chains

The chains are held together by disulfide bonds (strong chemical links between sulfur atoms). The carbohydrate chains are attached to the heavy chains.

Because the antibody is made of four separate polypeptide chains assembled together, it has a quaternary structure — this just means its shape comes from more than one polypeptide chain joined together.

1. The Variable Region

• This is found at the tips of the Y — the top ends of both arms.
• The shape of this region varies between different antibodies.
• It contains the antigen-binding site — the precise spot where the antibody grips onto a foreign substance.
• The antigen-binding site is made up of about 110–130 amino acids from both the light and heavy chains.
• Because the amino acid sequence differs between antibodies, the shape of the binding site differs too — giving each antibody specificity (meaning it can only fit one particular antigen, like a key fitting one lock).

How Structure Relates to Function

What Do Antibodies Actually Do?

Antibodies fight pathogens (disease-causing organisms) in several ways:

1. Neutralisation of viruses — Antibodies bind to the surface of a virus, blocking it from attaching to a host cell's receptors. This prevents the virus from entering the cell and causing an infection.

2. Neutralisation of toxins (acting as antitoxins) — Bacteria sometimes release poisonous chemicals called toxins. Antibodies can bind to these toxins and neutralise them, stopping them from damaging your body's cells.

3. Opsonisation — Antibodies coat the surface of a pathogen. This "flags" or marks the pathogen so that phagocytes (white blood cells that eat pathogens) can recognise it. The phagocyte has receptors that grip the constant region of the antibody, making it much easier to engulf and destroy the pathogen.

4. Agglutination — Some antibodies (especially a type called IgM, which exists as a pentamer — five antibody units joined together, giving ten binding sites) can attach to antigens on multiple pathogens at the same time. This causes the pathogens to clump together. The clump is easier for phagocytes to engulf, and it also stops pathogens from spreading through the body.

5. Immobilisation of pathogens — Antibodies can bind to the flagella (whip-like tails) of bacteria. When these are blocked, the bacteria cannot move properly, making them easier for phagocytes to capture.

6. Complement activation and lysis — Antibodies bound to a pathogen can trigger a group of blood proteins called complement proteins. These proteins punch holes in the pathogen's cell membrane. Water then rushes in by osmosis (the movement of water across a membrane), causing the cell to swell and burst (lyse).