Recognition of antigen by B cells and antibody

• Describe antibody structure

  Consist of two identical light chains and two identical heavy chains, which are bound by a disulphide bond and non-covalent interactions such as hydrogen bonds and hydrophobic interactions to form a heterodimer. The aminoterminal domains bind to antigens and the variable regions combine to form an antigen binding site. The hinge region allows for flexibility and some independent movement between the two Fab arms, which allows the antibody to bind to sites that are far apart.

  
• Describe antibody cleavage by papain

  Papain cuts at the disulphide bonds that link the heavy chains, which leads to the formation of two Fab fragments and one Fc fragment.

  • Fab: Fragment antigen binding, can bind antigens
  • Fc: Fragment crystallisable, cannot bind antigens

  
• What is the antigenic epitope?

  This is the part of the antigen that is recognised by antibodies and each antigen typically has multiple epitopes.
• Describe the difference between linear and conformational epitopes

  Conformational epitopes consist of discontinuous amino acid sequence that are brought together by protein folding in its naive state. Whereas, linear epitopes consist of amino acids located continuously along the protein backbone structure. Majority of antibodies recognise conformational epitopes.

  
• Describe the forces involved in antigen-antibody binding

  These interacts are usually non-covalent, such as hydrogen bones and Van der Waals forces, which are weak compared with covalent bonds. Therefore, a large number of interactions are required in order to form a strong interaction, which requires close proximity and a specific fit.
• Describe what affinity of antibody binding is

  This refers to the strength of the binding between an epitope and an antibody antigen binding site. The better the fit between the antigen and then antibody, the stronger the interactions and the higher the affinity. Antibodies can bind to different antigenic epitopes, but with variable affinities, this is known as crossreactivity.
• Describe what the avidity of antibody binding is

  This refers to the overall strength of an antibody-antigen complex. If an antibody uses both binding sites to bind to 2 epitopes on the same antigen then this will increase the total binding strength or avidity.

  
• What is antibody dependent cell mediated cytoxicity?

  This occurs when host cells are infected by some viruses and express viral proteins on their surface, which can be recognised by antibodies against that particular virus. Cells with the Fc receptor, such as macrophages, can bind to the antibody and kill the target cell by releasing perforin and granzymes.
• Describe the application of antibodies in immunotherapy

  Construct antibodies that can bind to tumour cells and therefore tag them for degradation by phagocytes. This stops binding of T cell brake proteins, which act as brakes for the immune response. Once these brakes are removed, the immune system can attack the cancer cells. Examples of brake proteins include CTLA-4 and PD-1.
• Describe the application of antibodies in blood typing

  Antibodies can immunoprecipitate antigens, leading to agglutination. Therefore, A and B antigens can agglutinate their respective complementary RBC antigens.

  • AB: agglutination with both A and B antibodies
  • A: agglutination with A antibodies
  • B: agglutination with B antibodies
  • O: no agglutination with either A or B antibodies
• Describe the different antibody classes

  • IgG: most abundant with IgG1 being the most common
  • IgM: earliest antibody made after antigen contact and forms a pentamer via disulphide bonds. This creates a complex with up to 10 binding sites.
  • IgD: rare with unclear function in serum
  • IgA1: serum and major Ig in mucosa with 2 subclasses.
  • IgE: lowest level in serum and produced by mast cells. Important I allergy and worm infection.
• Describe the structure of pentameric IgM

  IgM can form a pentamer, usually in the bloodstream but not in tissues. This increases its avidity for antigens before undergoing affinity maturation.

  
• Describe the structure of IgA dimers

  Constant regions of IgA and IgG contain a cysteine residue that is essential for polymerisation. The J chain allows polymerisation by linking to the cysteine of this constant region. This increases its avidity for antigens before affinity maturation.

  
• Describe IgA

  This is the predominant immunoglobulin in external secretions, such as saliva, tears and mucus. IgA serves an important function at membrane surfaces, as this is the main entry site of pathogens. IgA exists as a dimer in secretions.