Antibodies (OCR A Level Biology)

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 meaning 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
• The function of antibodies is to destroy pathogens within the body either directly, or by recruiting other immune cells
• Antibodies can act as anti-toxins, opsonins and agglutinins