Antibodies (Cambridge (CIE) A Level Biology): Revision Note

Antibodies: structure & 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

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 can create holes in the cell walls of pathogens causing them to burst, or lyse, when water is absorbed by osmosis