Structure of Antibodies

• Antibodies


  The products of activated B-cells (plasma cells) and are produced in response to infection during the adaptive immune response
• Antibodies


  • Y-shaped molecules, made of four polypeptide chains
  • Heavy (H) chains (50 KDa each) with four structural domains, three constant and one variable
  • Light (L) chains (25 KDa each) with one constant domain and one variable domain
  • Linked by disulphide bonds
  • Molecular weight of 150,000
• Variable region (Fab)


  Recognises specific antigen
• Constant region (Fc)


  Binds to Fc receptors on cells of the immune system that can destroy the pathogen expressing the antigen on its surface
• Antibody recognition and binding

  1. Highly specific sequence in the Fab variable region is complementary to a single antigenic determinant, or epitope, of an immunogenic protein that binds in the cleft between VH and VL

  2. Pairing of the first variable domain of one heavy chain and one light chain makes the variable region

  3. Two antigen binding sites on each antibody

  4. Diversity between antibody molecules allows each antibody to recognise one specific epitope
• Antibody repertoire


  • Collection of different antibodies within an individual, consisting of at least 10^11 different molecules
  • Can recognise all classes of molecules including proteins, polysaccharides and lipids
• Constant region (Fc)


  Interacts with other parts of the immune system to destroy the pathogen once the antibody is bound to it
• Antibody isotypes


  • IgM
  • IgD
  • IgG
  • IgA
  • IgE
• IgM


  • Low affinity of binding, but in the secreted form forms pentamers with a total of 10 antigen binding sites
  • Usually confined to the blood
  • First antibody to be synthesised during an immune response
• IgG and IgE


  • Monomers in the secreted form
  • IgG is the main isotype found in the blood
  • IgE is present in low levels in the blood
  • Can diffuse out of the blood and into tissues
• IgA


  • Usually found as a dimer in the secreted form
  • Specialised transport system at epithelial surfaces means that IgA is the main isotype found in secretions such as saliva, tears and mucus in the airway
  • Monomers are joined by the J-chain
• IgD


  Little IgD is produced at any time
• Transcytosis of IgA


  1. IgA antibody is transported across epithelia into external secretions by the poly-Ig receptor, a specialised transport protein

  2. Most IgA is synthesised by plasma cells lying just underneath the epithelia of the gut, airways, tears and salivary glands and mammary gland

  3. The IgA dimer is bound by the J-chain and diffuses and binds to the poly-Ig receptor on the basal aspect of epithelial cells

  4. The bound receptor-IgA complex undergoes endocytosis into a vesicle and transport, or transcytosis, across the epithelial cell to the apical surface

  5. At the apical surface the receptor is cleaved to leave the extracellular IgA-binding component, called the secretory component, bound to the IgA dimer

  6. This form of IgA is known as secretory IgA and is found in all external secretions and is important in mucosal defence