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.
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 and there are five types. It is these heavy chains which enable antibdodies 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 though disulfide bonds, they are then connected to a light chain by disulfide bonds as well. This therefore creates two identical antigen-binding sites.
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. 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. 
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.
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.
- ↑ The Editors of Encyclopaedia Britannica (2014) Antibody http://www.britannica.com/EBchecked/topic/27783/antibody (last accessed 19/11/14)
- ↑ 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)
- ↑ Weir, D. and Spencer, J. (1997) Immunology, 8th edition, page 7 New York: Churchill Livingstone
- ↑ Janeway CA, Shlomchik MJ, Travers P,&amp;amp;amp;amp;amp;amp;amp;amp;nbsp;Walport M. Immunobiology: The Immune System in Health and Disease: 5th ed. New York, NY: Garland Science; 2001.
- ↑ Fanger M, Lydyard P, Whelan A. Immunology: 3rd ed. New York, NY: Garland Science; 2011.
- ↑ Novimmune. Antibodies [Internet]. Geneva, Switzerland; Novimmune SA; 2015 [cited 2015 Nov 20]. Available from: http://www.novimmune.com/science/antibodies.html
- ↑ 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)