Surely during the last months in which we have been mired (and continue) by the Coronavirus pandemic, you will have heard a lot about antibodies and how they somehow allow us to detect whether or not a person is infected by Covid-19 , but do you know what exactly antibodies are? We now explain what they are for, how they work and also the different types of antibodies that exist.
Antibodies: what are they, types and how do they work
Antibodies, also called immunoglobulins , represent glycoprotein molecules particularly involved in the immune response . They are produced by B lymphocytes and constitute the only class of molecules capable of offering a specific response to external agents ( antigens ) that have settled in the host. Simplifying, it can be stated that immunoglobulins, by binding to the antigen in a specific way, favor its subsequent elimination.
General characteristics of immunoglobulins
There are five isotypes of immunoglobulins: IgM, IgA, IgE, IgG, IgD, all with a common basic molecular structure that gives them the classic “Y” shape, a fundamental element for the recognition of foreign organisms (viruses, toxins, bacteria, etc. .).
This structure consists of a fixed region and variable regions located in correspondence with the lateral arms of the “Y”, within which specific binding sites for antigens can be recognized. Immunoglobulins are symmetric and are made up of four chains : two identical light chains and two heavy chains that contain a series of repeating homologous units that fold to form a structure called the “Ig domain”.
Function and mechanism of action of antibodies
Each side arm of the “Y” has specific binding sites that block the antigen , allowing the antibody to be activated. Activation triggers the reproduction of a large number of B lymphocytes that differentiate into plasma cells, responsible for the production of thousands of other antibodies . However, the antigen can be attacked differently depending on the immunoglobulin isotype.
Classification of antibodies
As we have already mentioned, there are five immunoglobulin or antibody models, similar in general molecular structure, but different in terms of functions and modes of action.
- IgG – immunoglobulin G : They represent the most representative class of immunoglobulins from a numerical point of view (70-75%) and the most produced isotype in the course of the immune response. They are very effective opsonins and have the function of stimulating phagocytosis of pathogens by phagocytes and activating the complement system (a nonspecific defense mechanism of the immune system). IgGs are the only isotype capable of responding effectively to bacterial toxins and acting as a protective element against the fetus during pregnancy. They have a half-life of between twenty-one and twenty-eight days.
- IgA: immunoglobulin A: Most frequently distributed in body secretions, they are the most common isotype in the mucous membranes of the digestive and respiratory tracts, although they are also present in breast milk, saliva, and bile. They perform the fundamental functions of immunity and neonatal immunity at the level of the mucous membranes.
- IgE – immunoglobulin E: They participate in the immune response to parasites, but play a fundamental role in the course of allergic reactions.
- IgM: immunoglobulin M : This histotype has a half-life of approximately 5 days and occurs in two ways: the first is a monomeric form associated with the membrane of B lymphocytes that functions as a receptor, the second is a pentameric form in which The five units of antibodies are held together by a small protein called the J chain, this structure allows the immunoglobulin to have 10 binding sites for the antigen. They constitute 10% of all immunoglobulins.
- IgD: immunoglobulin D : They represent the smallest portion of all the isotypes described so far, about 1%. They are found only on the surface of mature B lymphocytes and have the sole function of activating lymphocytes and promoting differentiation into plasma cells.