Antibodies

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An antibody, otherwise known as an immunoglobin, is a Y-shaped protein produced by the body's immune system when foreign objects are detected.The antibody recognises a specific site on the foreign target called antigens.

The antibody then binds to the antigens in order to remove them from the body. A wide range of substances are regarded as antigens, such as toxic materials like insect venom[1].

However, antibodies can also be produced when healthy tissue is mistaken for a harmful substance by the immune system. This is known as an autoimmune disorder[2].

Antibody-antigen interactions can also explain certain processes such as the ABO blood group system[3].

Contents

Structure of Antibodies 

Antibodies are proteins composed of subunits of amino acids. The basis of the bivalent antibody molecule are four polypeptide chains, within which are two types, referred to as light (classed as either lamda or kappa) and heavy chain. The light chain is around 25 kDa,and the heavy chain is 50 kDa. The heavy chain is important as the class of the antibody is derived from its
Structure of an Antibody

structure and there are five types. It is these heavy chains which enable antibodies to be separated into the five classes (or isotypes) of immunoglobins; IgG, IgA, IgD, IgM and IgE. The way in which the two heavy chains are connected to one another is through disulfide bonds, they are then connected to a light chain by disulfide bonds as well. This therefore creates two identical antigen-binding sites[4].

The regions of the heavy chain includes three constant domains, which are the same for all antibodies, and the one variable domain. The light chains are also composed of a constant domain and a variable domain. The variable regions of the antibodies are where the antigen binding site is located and the constant domain is responsible for the outcome of the antigen[5]. The Fc region is the part of the antibody that binds to cell surface receptors on cells such as macrophages. Finally, the Fab region is primarily known as the location for the antigen binding site[6]. The domains of antibodies are around 110 amino acids in length.

The five classes of immunoglobulins have different functions in order to protect the body. IgA is present on the membrane of mucosal epithelial cells and is then released in fluids such as saliva and tears. This immunoglobulin protects the mucosal surfaces by, for example, binding to microorganisms to prevent them from attaching to the mucosal surfaces. IgG is the main antibody and has many roles including the promotion of phagocytosis of invading pathogens. IgM is especially effective at binding to antigens on pathogens and causing agglutination to prevent them from infecting other body cells. Immunoglobulin M is present on the membrane of B-cells. IgE are involved in causing inflammation when an allergen enters the body (allergic reaction) and does this by stimulating mast cells which leads to the release of mediators. Finally, IgD is also present on B-cells and is thought to work with IgM[9].

Antigen-antibody interactions

The antigen-antibody interactions are formed by a combination of bonds, such as hydrogen, van der Waals and electrostatic forces. Affinity is used to measure the strength of the interaction between an epitope and the antigen-binding site. It can be defined by the affinity constant KA, whereby it describes the extent of how much of the antigen-antibody complex exist when equilibrium is reached. The time it takes depends on factors such as diffusion rate. Therefore, high-affinity antibodies will bind a larger amount of antibodies, in contrast to low-affinity antibodies. As a result, the affinity of monoclonal antibodies is measured more accurately (compared to polyclonal antibodies as they are selective for only one epitope. Avidity measures the overall strength of 3 factors: affinity of the antibody to epitope, antibody and antigen valency and the interactions of the structural arrangements. As antibodies are multivalent, the interactions help enhance their stability as high avidity structures are being formed[7].

Origin of Antibodies 

Antibodies are produced by plasma cells which have originated from B lymphocytes

There is a complementary antibody to every antigen because of the variability in structure of the different classes and subclasses of antibodies[8]

References

  1. The Editors of Encyclopaedia Britannica (2014) Antibody http://www.britannica.com/EBchecked/topic/27783/antibody (last accessed 19/11/14)
  2. David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, David R. Eltz, and Stephanie Slon. (2012) Antibody.http://www.nlm.nih.gov/medlineplus/ency/article/002223.htm (last accessed 19/11/14)
  3. Weir, D. and Spencer, J. (1997) Immunology, 8th edition, page 7 New York: Churchill Livingstone
  4. Janeway CA, Shlomchik MJ, Travers P, and nbsp;Walport M. Immunobiology: The Immune System in Health and Disease: 5th ed. New York, NY: Garland Science; 2001.
  5. Fanger M, Lydyard P, Whelan A. Immunology: 3rd ed. New York, NY: Garland Science; 2011.
  6. Novimmune. Antibodies [Internet]. Geneva, Switzerland; Novimmune SA; 2015 [cited 2015 Nov 20]. Available from: http://www.novimmune.com/science/antibodies.html
  7. Bio-Rad. Avidity and affinity of antibodies. 2016. Available from: https://www.bio-rad-antibodies.com/antigen-antibody-interactions.html (Last accessed 18/10/16)
  8. Structure of an Antibody image. Avaliable on: http://www.biology.arizona.edu/immunology/tutorials/antibody/structure.html

9. 1. Arthur Rabson AR. Ivan M.Roitt IM. Peter J.Delves PJ. Really Essential Medical Immunology. 2nd Edition. Oxford: Blackwell Publishing Ltd. 2005.  

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