Antibodies, also known as immunoglobulins, are a group of structurally related glycoproteins that play a crucial role in the immune system. Produced primarily by B-lymphocytes, antibodies have the ability to recognise and neutralise antigens, which are foreign substances in the body, such as bacteria, viruses and toxins.
Structure and Function of Antibodies
Antibodies can exist in two main forms: membrane-bound or secreted. Membrane-bound antibodies act as receptors that initiate the activation of B-lymphocytes upon binding to an antigen. On the other hand, secreted antibodies play a key role in humoral immunity by activating various effector mechanisms to eliminate bound antigens.
One of the most salient features of antibodies is their great diversity. The antigen-binding regions on antibody molecules are highly variable. This allows an individual to be able to generate millions of different antibodies, each with specificity for a particular antigen. This diversity is essential for an effective adaptive immune response.
All antibodies have a common structure consisting of two identical light chains and two heavy chains. Each chain consists of Ig domains (of approximately 110 amino acids) containing conserved sequences and intra-chain disulphide bonds. It is the V regions of the light and heavy chains that vary between antibodies and are responsible for antigen specificity.
Antibodies are divided into several isotypes and subtypes according to differences in the C regions of the heavy chains. These isotypes include IgM, IgD, IgG, IgE and IgA, each with distinct functional properties. In addition, the two light chains of an antibody are of the same isotype (κ or λ), which are differentiated by their C domains.
Most of the effector functions of antibodies are mediated through the C-regions of the heavy chains, although these functions are triggered by antigen binding to the binding site in the V-region. This binding can trigger a variety of immune responses, such as pathogen neutralisation and activation of the complement system.
Monoclonal antibodies are antibodies that are derived from a single B-lymphocyte clone and have specificity for a single antigenic determinant. These antibodies can be produced in the laboratory and are widely used in research, diagnosis and therapy. Their ability to recognise a single antigenic structure makes them powerful tools in modern medicine.
Antigens are substances that specifically bind to antibodies or antigen receptors on T-lymphocytes. These antigens include a wide variety of biological molecules, such as sugars, lipids, proteins and nucleic acids. In contrast, T-lymphocyte receptors generally recognise only antigenic peptides.
Macromolecular antigens contain multiple epitopes or antigenic determinants that can be recognised by antibodies. Linear epitopes consist of adjacent amino acid sequences, whereas conformational determinants are formed by the folding of the polypeptide chain.
The affinity between an antibody and its epitope is usually represented by the Kd value. Polyvalent antigens have multiple identical epitopes to which several antibodies can bind. In the case of IgM, up to 10 antibodies can bind simultaneously to the same antigen, which increases the avidity of the antibody-antigen interaction.
During an immune response, antibodies can undergo structural changes. Initially, B cells produce membrane-bound antibodies. However, as B cells are activated and become plasma cells, they begin to secrete antibodies with the same antigenic specificity. Changes in the C regions of antibodies without altering the V regions are the basis of isotype switching. This allows the effector function of the antibody to be modified without changing its specificity. In addition, point mutations in the V-regions lead to an increase in affinity for the antigen, a process known as affinity maturation.
Antibodies are essential molecules for the body’s protection against pathogens, and their structural and functional diversity is key to the success of the adaptive immune system. From their production in B lymphocytes to their application in medicine and research, antibodies continue to play a crucial role in our defence against disease and in the development of innovative therapies.