Immunity: Question 2

Worked solution

Part (a): Structure of an antibody

An antibody (immunoglobulin) is a protein built from four polypeptide chains: two identical long chains, called heavy chains, and two identical short chains, called light chains. These four chains are held together (partly by disulfide bonds) into a molecule with a characteristic Y shape.

Each of the four chains has two regions:

• A constant region, which has a similar amino acid sequence in all antibodies of a given class.
• A variable region, which has an amino acid sequence, and therefore a shape, that differs between antibodies.

The variable regions of one heavy chain and one light chain combine at each of the two tips (arms) of the Y to form an antigen-binding site, so each antibody molecule has two identical antigen-binding sites.

Part (b): Why an antibody is specific to one antigen

The amino acid sequence of the variable region folds into a particular three-dimensional shape. This shape is complementary to the shape of one specific antigen (or a specific region of it), so the antigen-binding site and the antigen fit together in the same way a key fits one specific lock.

Because different antibodies have variable regions with different amino acid sequences, and therefore different shapes, each antibody can bind only to the one antigen whose shape is complementary to its own antigen-binding site. This is why a given antibody does not bind to unrelated antigens.

Part (c): How the antibodies help destroy or remove the bacterium

Agglutination: Since each antibody molecule has two antigen-binding sites, it can attach to antigens on the surface of two separate bacterial cells at the same time. With many antibody molecules present, this cross-links bacteria together into large clumps. Clumped bacteria are less able to spread through the body, and a phagocyte can engulf and destroy many bacterial cells at once rather than tackling them individually, making the immune response more efficient.

Opsonisation: Antibodies bind to antigens on the surface of the bacterium, coating it. This antibody coating acts as a marker (an opsonin) that phagocytes can recognise and bind to more readily, which increases the rate and efficiency with which the phagocytes engulf and digest the bacterium by phagocytosis.

(A third valid way, not required here, would be neutralisation: if the bacterium produces a toxin, antibodies can bind to and block the active region of the toxin molecule, preventing it from harming the patient’s cells.)

Final answers

• (a) An antibody is a Y-shaped protein of two heavy and two light polypeptide chains, each with a constant region and a variable region; the variable regions form two antigen-binding sites.
• (b) The variable region’s specific 3D shape is complementary to one antigen only, so the antibody binds specifically to that antigen.
• (c) Agglutination (cross-linking bacteria into clumps via the antibody’s two binding sites) and opsonisation (coating the bacterium to mark it for more efficient phagocytosis).