An antibody, otherwise known as an immunoglobin, is a Y-shaped protein produced by the body's immune system when foreign objects are detected. Antibodies are secreted by B cells during an immune response and comprises the majority of the humoral immune response. A b cell excretes vast array antibodies that are similar in structure but not identical. The antibody recognises a specific site on the foreign target called antigens.
Structure of AntibodiesAntibodies are composed of subunits of amino acids. The basis of the bivalent antibody molecule are four polypeptide chains, within which are distinguished into two types, referred to as light (classed as either lambda or kappa) and heavy chain. The light chain is around 25 kDa, and the heavy chain is 50 kDa. There are five classes of antibodies, and therefore the heavy chain is important for determining which class the immunoglobulin belongs to
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 disulphide bonds, they are then connected to a light chain by disulphide 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 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.
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 the structure of the different classes and subclasses of antibodies.
Klaus DE. Immunology: Understanding the immune system. 2nd edition. John Wiley and Sons Ltd. 2009
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