Antibodies (immunoglobulins) are synthesized and secreted by plasma cells; they are glycoproteins which play a vital role in regulating the immune system, accurately identifying specific antigens originating from pathogens and toxins. Once an antibody recognizes and binds to its corresponding antigen, it will activate the immune response to neutralize and eliminate pathogens.
The use of antibodies in life science and molecular research is comprehensive. For instance, antibodies are used in immunoprecipitation to recognize and isolate specific proteins. Antibodies’ ability to bind specific antigens can also be used to examine the interaction between specific proteins (co-immunoprecipitation, or Co-IP). Other techniques which make use of antibodies include ELISA (enzyme-linked immunosorbent assay) and Western blotting for protein detection. Once the labeled antibody binds to the target antigen, a detectable signal is produced in a subsequent enzymatic reaction, quantifying the proteins. Further methods which take advantage of antibodies’ properties include flow cytometry and cellular imaging.
In general, antibodies exist as two primary forms: a soluble form and a membrane-bounded form. Soluble antibodies are secreted by plasma cells into the bloodstream, whereas the membrane-bound form acts as a receptor on the membranes of B cells. Once an antigen is bound to the B-cell receptor, it can generate and transduce signals to activate B-cells and start an immune response against the invading microbes.
Typically, an antibody is made up of four polypeptide chains, consisting of two heavy and light chains. The heavy chains connect to both each other and the light chains via disulfide bonds. There are five types of heavy chains for humans: α, δ, ε, γ, and μ, which are used to classify each isotype: IgA, IgD, IgE, IgG, and IgM respectively (i.e. Ig standing for immunoglobulin, A stands for α).
Each polypeptide chain is made up of 70-110 amino acid-long immunoglobulin domains. Variability within these domains exists at one end of the polypeptide chain, which confers the ability to bind to specific antigens (called variable domain, VH for the heavy chain and VL for light chain). The rest of the domains contain constant amino acid sequences (on both the light and heavy chains) and are simply known as the constant region. They ultimately define the isotype of the antibody.
The overall structure of the antibody consists of two parts: the Fragment antigen binding (Fab) region and the Fragment crystallizable (Fc) region. The Fab region consists of both variable and constant domains that are responsible for binding to antigens, while the Fc region only has constant domains and is in charge of mediating immune responses by binding to the Fc receptors on immune cells and other immune-regulating molecules such as complement proteins.
Antibody Isotypes and their Roles
As mentioned above, the isotype of the antibody is defined by its heavy chain class. IgD, IgE, and IgG exist as monomers while IgA and IgM are in the dimer and pentamer form respectively. In humans, there are five main antibody isotypes and each isotype contains specific structure, locations, and functions in mediating the immune response of host organism.
• IgA antibodies are commonly found and produced in the mucosa (membranes that secrete mucus, such as the stomach, trachea or mouth cavity ) where they bind to invading pathogens, facilitating their elimination.
• IgG mostly contribute to host humoral immunity as they can; bind to and cause clamping of pathogens (agglutination), neutralize toxins and activate the complement system (proteins that trigger inflammation). IgG can also cross through the placenta to provide immunity to the fetus.
• IgM, which can either be expressed on the surface of B-cell (monomer) or exist as a secreted form (pentamer), has very high avidity for antigens, helping to eliminate pathogens in the early stages of infection before there are sufficient levels of IgG.
• The actual function of IgD remains unknown but possibly works with IgM to mediate B-cell activation and development.
• IgE plays a major role in triggering an allergic reaction and protects the host by attacking invading parasites.
Medical applications of Antibodies
In a clinical aspect, antibodies can be used for a broad range of disease treatments as they can be modified to bind to specific antigens or molecules. Moreover, the structure of antibodies can be amended to make them capable of carrying drug molecules or radioactive particles into certain cells for targeted therapy.
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