What Are Antibodies?
Antibodies are proteins produced by our immune system, mainly the plasma cells, in response to foreign substances. Each antibody attaches itself to only one antigen. The main purpose of antigen-antibody interaction is the destruction of antigens. Antibodies can destroy antigens or make it easy for the immune system to destroy them.
What Is the Structure of an Antibody (Immunoglobulin)?
An antibody is also known as immunoglobulin. It has a Y-shaped structure and consists of four polypeptide chains. Two are heavy chains, and the other two are light chains. The Y-shaped structure helps the antibody bind antigens and perform their biological functions. The antibody has a fragment antigen-binding site (Fab) and fragment crystallizable region (Fc). Fab, or the fragment antigen-binding site, is the region that attaches to the antigens. Fc, or the fragment crystallizable regions, is the end region on the antibody that interacts with the Fc receptors, which are the receptors present on the surface of the cells. The immune system gets activated due to Fc site binding to the Fc receptors.
What Are the Different Types of Antibodies?
Antibodies are classified into different types based on their H-chain (heavy chain). Each antibody is provided a letter. The abbreviation of immunoglobulin, “Ig”, accompanies the alphabetical letter. The antibodies are IgA, IgG, IgM, IgE, and IgD.
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IgA:
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It is mainly present in the saliva, tears, mucus membranes of the intestinal, nasal, and genital tract, breast milk, colostrum, and respiratory tract secretions.
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It makes up about 10 to 15 percent of all immunoglobulins.
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It differs in molecular structure and appears in two forms. Monomeric IgA is also known as serum IgA, and dimeric IgA is known as secretory IgA.
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It acts as the first line of defense against ingested and inhaled microbes and protects the mucosa of the digestive, respiratory, and genitourinary systems.
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It also helps to activate the complement system by activating the alternative pathway.
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IgG:
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It comprises 70 to 75 percent of all the immunoglobulins present in the blood.
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It is produced as a secondary response to pathogens.
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It activates the complement system and plays a major role in providing protection.
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IgG is divided into four types depending on the molecular size and position of the disulfide bonds. The four subtypes are IgG1, IgG2, IgG3, and IgG4.
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IgG1 and IgG3 production is triggered by proteins. The presence of foreign polysaccharides triggers IgG2 and IgG4 production.
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Since IgG is abundant, it forms a major part of humoral immunity (immune reaction targeting a particular antigen).
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IgG2 plays a role in the defense mechanisms against capsulated bacteria.
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IgG is found in abundance in the extracellular fluid.
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Because of the small size of the immunoglobulin, IgG can cross the placenta and protects the fetus and the baby by providing passive immunity. It is the only immunoglobulin that can cross the placenta and is found abundantly in newborns.
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IgG also initiates the process of phagocytosis and causes an opsonization reaction.
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IgM:
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It is the first formed antibody in response to a foreign substance or antigen. It is the largest antibody present in the body.
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It constitutes five percent of all immunoglobulins.
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It only exists in the polymeric form and is a pentamer that is it is composed of five subunits.
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Since it is large, it has a lower attachment affinity for antigens; however, due to its large size, it has a higher strength of binding than the other antibodies. It also activates the complement system.
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IgE:
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It exists in monomeric form (it can bind together to form a polymer).
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IgE protects the body against parasitic infection. However, it binds to the receptors in the mast cells and basophils and causes allergic reactions.
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It is present in a very small concentration in the serum. Therefore, during allergic reactions, the concentration of IgE increases, which increases the eosinophilic count at the site of the allergic reaction.
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It is present in the respiratory and intestinal tract cell lining.
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IgD:
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It is a monomeric antibody present in a minor amount in the blood serum.
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It also helps in the production of antibodies by lymphocytic differentiation.
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It constitutes less than a percent of the total antibody content of the body.
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What Are the Functions of Antibodies?
Antibodies are produced by the B-cells that act specifically on a single antigen. It contributes to immunity mainly in three manners: neutralization (preventing the pathogenic entry or damaging the cellular structure of the pathogens by binding to them), opsonization (covering the pathogens and thus facilitating their removal), and phagocytosis (engulfment of the bacteria and their destruction).
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It attaches to pathogens and causes activation of the immune system.
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It protects against viruses.
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It helps in phagocytosis and ingests the bacteria present in the body.
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Since the antibodies can remain active in the body even after years after antigenic exposure, it provides long-term protection against the antigens.
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It nullifies the bacteria's toxins and attaches to the antigens to quantify its efficacy.
What Is the Mechanism of Production of Antibodies?
When the body's immune system detects the presence of any foreign particles for the first time, the macrophages act on them and pass them to the B-cells of the immune system. B-cells respond through antibody production. The antibodies contain a paratope site (site on the antibodies that bind with the antigens) that binds with the epitope site (specific site on the antigen that binds with the antibodies) on the antigen. The B-cells produce a specific antibody against the epitope. Once these antibodies are released in the bloodstream, they attach themselves to specific pathogens and eliminate them from the body. The antibodies either directly attack the pathogens or bind to the surface of the pathogens and send signals to the immune so that they can be eradicated from the body. The antibodies rest in the body forever and protect if the pathogens try to re-enter the body again.
What Are Monoclonal Antibodies?
Monoclonal antibodies are the ones that are produced in the laboratory by humans through genetic engineering techniques. They have made a very valuable contribution to research and medicine. They can recognize antigenic sites on any molecules ranging from drugs, hormones, cell surfaces, and pathogens.
Conclusion
Antibodies protect our bodies from various pathogens and infections. Antibodies are proteins that are produced by our bodies naturally. However, with recent advancements, artificially produced monoclonal antibodies are used to treat various other health issues like cancer, rheumatoid arthritis, etc. Therefore, knowing the functional mechanism of antibodies will help understand the antimicrobial effect.
