Overview

Antibodies, or immunoglobulins (Ig), are glycoproteins produced by the immune system to aid recognition of invading microorganisms. They coat foreign but not self antigen, allowing the immune system to differentiate between these to mount appropriate responses. All immunoglobulins share the following basic properties:

  • Produced by B cells or plasma cells after contact with cognate antigen
  • Antigen-specific
  • Can be secreted (as a soluble protein) or membrane bound (as part of a B cell receptor complex)
  • Present in blood, tissue and lymph
  • Make up 20% of the plasma proteins
  • Constitute the gamma globulin fraction on electrophoresis of serum proteins

 

 

 

 

    There are 5 classes of human immunoglobulin molecules:

        • IgG
        • IgA
        • IgM
        • IgE
        • IgD

        (Useful mnenomic: GAMED)

        Each immunoglobulin class can by distinguished by a unique amino acid sequence in the Heavy chain Constant region (CH) which confers their class-specific structural and functional properties. This governs how they stimulate the innate system to remove antigen.

         

         

         

         



        Antibody Structure

        Figure 1. Basic antibody structure showing two heavy chains and two light chains.

        Immunoglobulins are the archetypal members of the Immunoglobulin Gene Superfamily: a large family of proteins that are related by sequence as a result of their common ancestry (paralogs). Some examples of this family are:

        (1) Multiprotein complexes:

        1. Antibodies IgG, IgA, IgM, IgE and IgD
        2. Major Histocompatibility Complex (MHC) molecules: Class I (beta-2 microglobulins) and Class II
        3. T-cell receptros (TCRs)

         

            (2) Single proteins:

            1. Poly-Ig: transports IgA across mucosal membranes
            2. Co-receptors to TCRs (CD4; CD8; CD3 gamma)
            3. CD90 (Thy-1) on T cells: role not fully known; thought to be involved in neurone growth and differentiation
            4. CD56 (N-CAM): adhesion molecules on neurons, NK cells and T-cells

             

              The members of this superfamily contain a conserved immunoglobulin domain of two beta sheets stabilised by a disulphide bond. This conserved domain, which stabilises proteins in the extracellular milieu, provides antibodies with long half-lives.

               

               

               

               

              All antibodies share a basic molecular H2L2structure:

              • 2 x LIGHT (L) (25 kDa), and
              • 2 x HEAVY (H) (50-70 kDa) chains

              which are joined together by intermolecular disulphide bonds

              The molecule itself is classically described as Y-shaped: a hinge region that fulfills the role of a 'flexible tether' links the Fc (no antigen-binding activity) and Fab (antigen-binding) regions. This flexibility allows independent movement of the two Fab fragments.

                   

                    1. Heavy Chains

                    There are 5 different classes of H chains that can be found in humans (designated by lower-case Greek letters)

                    • Mu (IgM)
                    • Delta (IgD)
                    • Gamma (IgG)
                    • Epsilon (IgE)
                    • Alpha (IgA)

                    2. Light Chains are of 2 types:

                    • Kappa
                    • Lambda

                    Both types of light chains can be found in all 5 classes, but a single immunoglobulin molecule can only contain one type of a light chain (i.e. it is either K-K or L-L)

                     

                    Immunoglobulin Domains.

                    Both Heavy (H) and Light (L) chains have repeating substructures known as domains.

                    • Each domain is approximately 110 amino acids long, stabilised by intrachain disulphide bonds
                    • H chains usually contain 4 or 5 domains, whereas L chains have only 2 domains
                    • The domains at the N-terminal of H and L chains are the Variable (V) domains (VH and VL, respectively)- this is where the amino acid sequence differs between clones to confer antigen specificity
                    • In contrast, Constant domains (CH and CL) on the C-terminal do not differ between antibodies within a class.

                     

                     

                     

                     

                      Antibody-Antigen Interaction

                      Hypervariable regions

                      Sequence variability is not evently distributed in VL and VH domains. Instead, variability is confined to three hypervariable regions on each chainThese regions are equally spaced between the framework regions that show less variability. Upon protein folding, the hypervariable regions are brought together at the N-terminus to form an antigen-binding site (paratope) that interacts with the antigen.

                      For this reason, hypervariable regions are also sometimes called the complementarity-determining regions (CDRs) because they are complementary in shape to an antigenic epitope.

                       

                      Diversity of immunoglobulin repertoire.

                       Variability is a term that is used to describe the generation of immunoglobulins of different specificities.This is essential to their functioning as a recognition molecule. The extent of variablity in paratope structure is such that antibodies exist which could recognise pathogens from other planets.

                      Variability is partly due to different combinations of H and L chain V regions. Furthermore, the unique DNA sequences of each V region are generated by expression of one of a number of possible genome encoded segments. This is known as V(D)J recombination, and results in a DNA sequence containig one of each of: 

                      1. V (for Variable)
                      2. D (for Diversity), and
                      3. J (for Joining) segments

                      The variability is also then further enhanced by the presence of Recombination Signal Sequences (RSSs) and, finally, faciliated by somatic hypermutation in mature B cells.

                       

                        Antibody-antigen interaction is based on reversible non-covalent forces that are weak unless antibody and antigen are in good proximity determined by complementary surfaces. Extremes of pH, high salt concentrations, detergents and high epitope concentrations can all disrupt the antibody-antigen interaction.

                        There are 4 types of molecular forces involved:

                          1. Hydrogen bonds
                          2. Electrostatic
                          3. Van der Waals
                          4. Hydrophobic forces

                                The contribution of each depends on the particular antibody and antigen involved (i.e. their amino acid composition). This determines the affinity (i.e. the strength) of antibody-antigen interaction.

                                 

                                 

                                 

                                 

                                Immunoglobulin classes

                                Selected properties of human immunoglobulins.

                                (Arranged in accordance with their corresponding serum concentrations: high->low)

                                 

                                1. IgG

                                IgG is the workhorse of the humoral immune response, with different subclasses produced to induce different types of immune effector function.

                                • Heavy Chain class: GAMMA
                                • IgG has the highest mean concentration in human serum: 7-18 mg/mL (the most abundant Ig class in serum). Its serum half-life is 21 days
                                • It has a monomeric structure and has 3 Constant domains in its H chain, CH1-CH3
                                • It activates complement (*) and crosses the placenta (**)
                                • It's major mode of action is by binding to Fc receptors on circulating phagocytes (e.g. neutrophils) for opsonisation of antigen
                                • IgG has 4 subclasses(in human, these are different in mice): these are closely related with 90-95% homology and are distinguished from one another by the size of hinge region and number and position of the interchain disulphide bonds between the H chains. Here they are numbered in accordance with their decreasing serum concentrations:
                                1. IgG1 (9 mg/mL) - most common, comprises ~70% of all serum IgG
                                2. IgG2 (3 mg/mL) - predominates in responses against polysaccharide antigens of encapsulated bacteria
                                3. IgG3 (1 mg/mL) - most effective complement activator
                                4. IgG4 (0.5 mg/mL) - does not appear to activate complement

                                 

                                  * The Complement cascade is one of the most powerful effector functions of antibody. Antibody is able to trigger activation of a group of serum proteins with several immune activities including cell lysis (proteins C5-C9), opsonisation (C3b), chemotaxis (C5a), inflammatory mediation (C3a-C5a), and clearence of immune complexes (C3b)

                                   

                                    ** Haemolytic Disease of the Newborn (Erythroblastosis Fetalis) is a disease which is caused by the unique ability of maternal IgG to cross the placenta to protect the foetus against infection.

                                    Erythroblastosis fetalis develops when a Rh-negative mother makes IgG antibodies when exposed to paternally inherited Rh-positive foetal erythrocytes during childbirth. During a subsequent pregnancy, anti-Rh IgG coats foetal erythrocytes which become destroyed in foetal liver by phagocytosis, resulting in haemolytic anaemia and toxicity due to excess bilirubin (manifested as jaundice).

                                    The disease can be prevented by sensitising the mother through administering her a preparation of anti-Rh IgG shortly after giving birth - this will remove foetal erythrocytes before they evoke maternal immune response

                                    • Major serum immunoglobulin: IgG=MaGor
                                    • IgG crosses placenta: Gestation

                                     



                                        2. IgA

                                        IgA is involved in protection of environmental/mucosal surfaces of the body and it has several interesting adaptations in line with this role.

                                        Heavy chain: ALPHA

                                        • The predominant mucosal antibody: IgA is able to cross epithelial cells with the help of Poly-Ig thereby neutralising antigen 
                                        • Mean concentration in serum is 0.8-4 mg/mL with a half-life of 7 days
                                        • Structure varies according to whether it is in serum (monomer: H2L2 unit similar to IgG); or in secretions (where it is found as a dimer of 2 x H2L2 + 1 joining J-chain* + Secretory Piece (SP)**. Trimers and tetramers can also be occasionally seen.

                                        J-chain is coded by the gene on a separated chromosome; reguired for proper polymerisation

                                        ** Secretory Piece is an additional protein that protects the molecule against proteolysis; is NOT made by a Ig-producing cell

                                        • Like in IgG, there are 3 constant domains in H chain
                                        • IgA does not classically activate complement, nor does it cross placenta
                                        • Usually found in serum
                                        • IgA is the major Ig found in SECRETIONS - e.g. mucus in intestinal and respiratory tracts; saliva; sweat; brest milk and colostum
                                        • Its presence in breast milk protects the newborn against infection during 1st month of life
                                        • It is also an important line of defence against bacteria (Salmonella; Vibrio Cholerae; Neisseria Gonorrhoeae) + certain viruses (polio, influenza) by inhibiting the adhesion of these to the epithelial lining
                                        • 'Neutralizing antibody'
                                        • IgA class switching is induced by rising TGF-Beta (Transforming Growth Factor-Beta) concentration
                                        • IgA has 2 subclasses: IgA1 and IgA2

                                         

                                           

                                          3. IgM

                                          IgM is a large, low affinity antibody which is produced by B cells as a first line of defence before class switching and somatic hypermutation have kicked in.

                                          • Heavy chain: Mu
                                          • Its mean concentration in serum is 0.4-2.5 mg/mL with a half-life of 7 days
                                          • IgM can be found as a monomer; expressed either as membrane-bound Ig on B cells, or as a pentamer (5 x H2L2) held together by a joining J-chain
                                          • In contrast to IgG and IgA, IgM has 4 constant domains in its heavy chains
                                          • It is able to activate the complement but does not crosses the placenta, however it is the first immunoglobulin to be generated by a neonate.
                                          • It is also the EARLIEST immunoglobulin to be produced after exposure to a novel antigen
                                          • Due to its pentameric structure, it has multiple antigen binding sites which results in overall high molecular valency- IgM plays an important role in the process of agglutination
                                          • IgM can also be transported across epithelium where is can block the intracellular pathogen binding and uptake
                                          • There are no subclasses of IgM

                                          1st immunoglobulin to appear: IgM=iMMediate

                                           

                                           

                                          4. IgD

                                          IgD is present on naive B cell surfaces, allowing them to recognise antigen at the induction period of adaptive immunity.

                                          • Heavy chain: DELTA
                                          • Its mean concentration in serum is less than 0.03 mg/mL with a half-life of 2 days
                                          • Monomeric structure only
                                          • Like IgG and IgA, IgD has 3 constant domains in their heavy chains
                                          • IgD does not activate complement or cross the placenta
                                          • It is highly unstable in serum and chiefly found on the surface of B cells rather than on its own
                                          • Thus, its main function is to act as a RECEPTOR for CELL ACTIVATION
                                          • IgD has no subclasses

                                           

                                           

                                            5. IgE

                                            IgE has adapted to deal with large, eukaryotic parasites. It is involved in allergy as an inappropriate anti-parasitic response.

                                            • Heavy chain: EPSILON
                                            • Its mean concentration in serum is less than 0.0005 mg/mL (NB IgE is the least abundant Ig in human serum) with half life of 2 days (NB IgE can have much longer half-life if surface-bound)
                                            • IgE is a monomer and has 4 constant domains in its heavy chains
                                            • It does not activate completment or cross the placenta
                                            • Despite its low concentrations in serum, IgE has a very high potency. It binds Fc receptors on MAST CELL and BASOPHILS, and thereby triggers ALLERGIC reactions - by activation of mast cells and subsequent release of HISTAMINE and other inflammatory mediators (TNF-Alpha; prostaglandins; leukotriens; platelet activating factor; and cytokines IL-4 and IL-13 to perpetuate TH2 response)
                                            • IgE is also involved in the inflammatory aspect of asthma
                                            • It is thought that IgE evolved to protect the host against metazoan parasite infections by high-jacking the TH2 (Helper T-cell) immune response to tissue damage.
                                            • Cytokines IL-4, IL-5, IL-9, IL-10 and IL-13 induce IgE class switching: they favor the development of Helper T cells (TH2) cells that, in turn, stimulate B lymphocytes to switch to IgE production
                                            • IgG4 inhibits IgE-FcRI interaction
                                            • IgE has no recognised subclasses

                                             

                                             

                                              Electrophoresis pattern of human serum proteins

                                              Electrophoresis pattern
                                              • Serum proteins can be resolved into 5 or 6 bands by electrophoresis
                                              • More than half of total human serum protein content can be accounted for by the immunoglobulin (gamma globulin) fraction

                                              Clinical significance: profound changes in globulin electrophoretic pattern almost always assumed to be due to changes in quantities of immunoglobulins - as they are most likely to affect the total globulin values.

                                               

                                               

                                              References/further reading

                                              Books:

                                              (1) Wood, P. (2001). Understanding Immunology. Ch. 3. Specific Immune recognition: the antibody molecule. Pearson Education Limited. Essex, UK.

                                              (2) Janeway, C.A., Travers, P., Walport, M. and Shlomchik, M.J. (2005). Immunobiology. The immune system in health and disease. Ch. 4 The generation of lymphocyte antigen receptors. Garland Science Publishing. New York. U.S.A.

                                              (3) Peakman, M. and Vergani, D. (2010). Basic and Clinical Immunology. Churchill Livingstone. Edinburgh. UK.

                                              Articles:

                                              (1) Schroeder, H. and Cavacini, L. (2010). Structure and function of immunoglobulins. Journal of allergy and clinical immunology. 125 (2): 41-52.

                                              (2) Butler, J.E., Zhao, Y., Sinkora, M., Wertz, N. and Kasckovics, I. (2009). Immunoglobulins, antibody repertoire and B cell development. Developmental and Comparative Immunology. 33(3): 321-333.

                                              Advertisement

                                              Fastbleep © 2019.