Antibodies (Cambridge International AS Biology)

Antibodies: Structure & Functions

Spec point — 11.2.1 · relate the molecular structure of antibodies to their functions

Antibody structure

• Antibodies are globular glycoproteins called immunoglobulins

• Antibodies have a Y-shaped quaternary structure with two ‘heavy’ polypeptide chains bonded by disulfide bonds to two ‘light’ polypeptide chains

• Antibodies have a constant region and a variable region

  • The constant regions do not vary within a class of antibodies but do vary between the classes.

    • The constant region determines the mechanism used to destroy antigens

  • The variable region is where the antibody attaches to the antigen to form an antigen-antibody complex

    • At the end of the variable region is the antigen-binding site.

    • Each antigen-binding site is generally composed of 110 to 130 amino acids and includes the ends of the light and heavy chains

    • Antigen-binding sites vary greatly, giving the antibody its specificity for binding to antigens

      • Antibodies bind to a region of the antigen called the epitope

• Antibodies also have a hinge region which gives flexibility to the antibody molecule

  • This allows the antigen-binding site to be placed at different angles when binding to antigens

Antibodies are Y-shaped molecules with specific antigen binding sites

Antibody function

• Antibodies are produced by B lymphocytes

• Their role is to bind to specific antigens

  • Antigens include parts of pathogens and their toxins, pollen, blood cell surface molecules and the surface proteins found on transplanted tissues

• Antibodies can function in several different ways

  • Antibodies can attach to viruses and to the toxins produced by pathogens to block them from entering or damaging cells

  • Antibodies can act as anti-toxins by binding to and neutralising toxins

  • Antibodies can attach to bacteria, making them readily identifiable to phagocytes; this is called opsonisation.

    • Once identified, the phagocyte has receptor proteins for the heavy polypeptide chains of the antibodies, which enables phagocytosis to occur

  • Antibodies can attach to the flagella of bacteria, making them less active; this makes it easier for phagocytes to do phagocytosis

  • Antibodies act as agglutinins, causing pathogens to clump together in agglutination

    • This reduces the chance that the pathogens will spread through the body and makes it possible for phagocytes to engulf a number of pathogens at one time

  • Antibodies bound to pathogens can activate complement proteins in the blood, which make holes in the cell surface membranes of the pathogens so that they burst (lyse)

Antibodies can act on pathogens in different ways