11.2.1 Antibodies

Structure

• Antibodies are globular glycoproteins called immunoglobulins
• Antibodies have a quaternary structure (which is represented as Y-shaped), with two ‘heavy’ (long) polypeptide chains bonded by disulfide bonds to two ‘light’ (short) polypeptide chains
• Each polypeptide chain has a constant region and variable region
• The constant regions do not vary within a class (isotype) of antibodies but do vary between the classes. The constant region determines the mechanism used to destroy the antigens
  • There are 5 classes of mammalian antibodies each with different roles

• The amino acid sequence in the variable regions of the antibodies (the tips of the "Y") are different for each antibody. 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 a site called the antigen-binding site. Each antigen-binding site is generally composed of 110 to 130 amino acids and includes both the ends of the light and heavy chains
• The antigen-binding sites vary greatly giving the antibody its specificity for binding to antigens. The sites are specific to the epitope (the part of the antigen that binds to the antibody)
• A pathogen or virus may therefore present multiple antigens different antibodies need to be produced
• The ‘hinge’ region (where the disulfide bonds join the heavy chains) gives flexibility to the antibody molecule which allows the antigen-binding site to be placed at different angles when binding to antigens
  • This region is not present in all classes of antibodies

A model of the generalised structure of an antibody molecule

Function

• Antibodies are produced by B-lymphocytes
• Antibodies bind to specific antigens that trigger the specific immune response. Every antigen has one antibody
• Antigens include pathogens and their toxins, pollen, blood cell surface molecules and the surface proteins found on transplanted tissues
• Antibodies are divided into five major classes (isotypes), each with a different role
• The function of antibodies differ:
  • Antibodies can combine with viruses and toxins of pathogens (e.g. bacteria) to block them from entering or damaging cells
  • Antibodies can act as anti-toxins by binding to toxins produced by pathogens (e.g. the bacteria that cause diphtheria and tetanus) which neutralises them making them harmless
  • 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, which makes it easier for phagocytes to do phagocytosis
  • Antibodies act as agglutinins causing pathogens carrying antigen-antibody complexes to clump together (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 (together with other molecules) can create holes in the cell walls of pathogens causing them to burst (lysis) when water is absorbed by osmosis

The functions of antibodies vary according to which type of antigen they act on