Role of Antibodies

• Antibody Functions


  • Inhibition / Neutralisation
  • Stimulation
• Inhibition of bacterial adhesion

  • Antibodies to bacterial adhesins prevent interaction with ligands on the epithelial cell surface and colonisation
  • Antibodies neutralise bacterial exotoxins, preventing them binding to specific receptors, entering and poisoning cells
  • Antibodies prevent virus binding to cell surface receptors, fusion of viral and cell membranes and entry into the cell
• Pathogens are widely distributed in the body and antibodies are equally widely distribute
• Antibodies are found in the blood, tissues and at the epithelial surfaces lining the gut and airway
• Antibody isotype


  Determines the distribution of antibodies, and antibodies of different types are adapted to function in different compartments
• Inhibition of bacterial adhesion to host cells

  • Bacteria colonise cell surfaces via specific cell surface molecules called adhesins, which allow them to bind to ligands on the surface of the host cell
  • Specific antibodies bind to the adhesins and prevent colonisation and uptake of bacteria
  • On their own, antibodies can not prevent bacteria replicating outside cells
  • IgA antibodies are especially important in preventing bacterial adhesion at mucosal surfaces, and IgG can protect cells within the body
• Neutralisation of bacterial exotoxins

  • Many bacteria, as well as venomous insects and snakes, mediate their effects by releasing cellular poisons or toxins, called exotoxins
  • A toxin must bind a specific molecule that acts as a cellular receptor which allows it to be internalised and poison the cell
  • Specific antibodies that bind to the receptor-binding site on the toxin and prevent its uptake are called neutralising antibodies and prevent the toxic effect
  • IgG can diffuse rapidly into tissues and neutralise toxins, whereas IgA is important at mucosal surfaces
• Inhibition of viral infection

  • Viral infection follows binding of virus to receptors on the cell surface, endocytosis, fusion of viral and cell membranes either on the cell surface or inside the cell which triggers entry of viral DNA
  • Antibodies to viral surface proteins inhibit viral infection
• Phagocytosis


  • Antibody promotes phagocytosis; IgG binds to Fc receptors on macrophages
  • Free IgG molecules do not bind to Fc receptors
• Complement C1 Activation


  • Antibody/antigen complexes activate complement C1 and enhance opsonisation with C3b
• Opsonisation


  • Antibodies can opsonise bacteria, enabling phagocytic cells to ingest and destroy the bacteria
  • Macrophages have receptors for the constant (Fc) region of antibodies on their surface and bind Fc and internalise it, together with the pathogen
  • All pathogens and particles that are bound by antibody are taken up by phagocytes for ingestion, degradation and removal from the body
  • IgG is the main opsonising antibody and aggregation of IgG on the bacterial surface allows binding to Fc receptors
  • Free IgG does not bind to Fc receptors, allowing the phagocyte to distinguish between the majority of free antibody and antibody that is bound to a pathogen
• Activation of complement


  • There are no Fc receptors for IgM
  • However, the complement system is activated by complexes of antigen with IgM and IgG antibodies, and is able both to opsonise and directly destroy bacteria
  • IgM is a powerful activator of the complement cascade
• The effector functions of antibodies, which are part of the adaptive immune system, also depend on cells (phagocytes) and molecules (complement) of the innate immune system
• The adaptive immune system has therefore developed to make the innate immune system more efficient
• Components and Actions of Complement

  • Lectin pathway
  • Alternate Pathways
  • Killing of pathogens by membrane attack complex C5b/C6/C7/C8/C9
  • Opsonisation of Pathogens – C3b
  • Recruitment of Inflammatory Cells – C3a, C5a
  • Classical Pathway
• All pathways depend on the formation of a C3 convertase that cleaves C3, the most abundant component of complement in plasma, to C3b, the opsonising component that is recognised by C3b receptors on phagocytes
• During this process pro-inflammatory components are formed, namely C3a and C5a, which recruit inflammatory cells such as neutrophils to help clear bacteria
• When C3b binds to and cleaves C5, C5b binds to bacteria and initiates the formation of the membrane attack complex, C5b/C6/C7/C8/C9, a lytic complex which creates a pore and destroys the integrity of bacterial cell walls
• Antibody Dependent Cell Mediated Cytotoxicity

  • Virus-infected cells are usually destroyed by CD8+ cytotoxic T-cells, alerted by viral peptides bound to surface MHC class I molecules
  • However, in the absence of MHC Class I, virus-infected cells can also signal the presence of intracellular infection by expressing viral proteins on their cell surface, that can be recognised by IgG antibodies
  • Cells bound by IgG antibodies can then be killed by natural killer cells (NK cells)
  • The destruction of antibody coated target cells is called antibody-dependent cell-mediated cytotoxicity and is triggered when antibody bound to the cell surface interacts with Fcγ IgG receptors on the NK cell
  • Cytotoxic attack involves the release of granules containing perforin to perforate the target cell, and granzymes which penetrate through the pores and induce cell death by membrane damage and/or apoptosis
• Activation of Eosinophils


  • Large parasitic worms (helminths) such as tapeworm become coated with specific IgE and IgA antibodies
  • Eosinophils have receptors for the Fc regions of both IgA and IgE (FceRII) and will use these to adhere to the tapeworm
  • Clustering of either IgA or IgE receptors activates the eosinophil triggering the release of reactive oxygen species and highly basic eosinophil granule contents to kill the tapeworm
  • Five molecules of IgA increase killing by 1,000 fold
• Immunological memory


  • After a B-cell has been activated it takes 4-5 days of clonal expansion before proliferation is complete and B-cells have become antibody-producing plasma cells
  • Theses cells have only a limited life span of 4 weeks, and once antigen is removed, most of the antigen-specific cells generated by clonal expansion die
  • However, some do persist after antigen has been removed and this is the basis of immunological memory
  • Immunological memory ensures a more rapid and effective response on a second exposure to the pathogen and therefore provides lasting immunity
• Comparison of primary and secondary antibody response

  • The secondary antibody response occurs after a shorter lag phase, achieves a higher plateau and produces antibodies of higher affinity
  • The intense response to A is seen in a mixture of A and B
  • The B antigen gives only a primary response
• Active immunisation


  • Involves immunisation with killed or heat-attenuated live organisms to induce a state of specific immunity
  • Attenuation makes the organism innocuous but still immunogenic
  • Life long protection can be achieved, especially if boosted by a repeat immunisation
• Passive immunity


  Achieved by the transfer of serum containing specific antibodies to an organism
• Innate defences in the airways

  • Mucus produced by submucosal mucus glands traps bacteria and prevents them binding to the epithelium
  • Cilia on the surface of epithelial cells beat in synchrony and sweep the mucus and trapped bacteria up the back of the throat where it is expectorated
  • The cough reflex
  • Airway macrophages on the surface of the epithelium recognise, engulf and digest bacteria
  • Antimicrobial factors, such as defensins and lysozyme are produced by airway epithelial cells
  • Activated macrophages produce cytokines that recruit neutrophils, another phagocytic cell, to help clear bacterial infection
• Adaptive immunity in the airways

  • Persistent infection leads to an increase number of plasma cells (mature B-cells) producing IgA in the tissue
  • IgA neutralises bacterial and viral proteins, preventing bacterial adherence and cytotoxicity
• ELISA


  1. Coat bottom of well with specific antibody, this is the capture antibody, recognises only protein X

  2. Add biological sample, a complex mixture of proteins, including protein X

  3. In separate wells, add known amounts of pure, standard, protein X to prepare a standard curve

  4. Wash away unbound proteins. Only protein X is recognised by the capture antibody, and remains bound to the plate

  5. Add second antibody. This is the detection antibody and is conjugated to an enzyme, eg peroxidase (*)

  6. Add the enzyme substrate and measure amount of coloured product in an ELISA plate reader at set wavelength. Compare absorbance with that of known amounts of protein X