Antibodies (Ab), also known as an immunoglobulin (Ig), are large, Y-shaped blood proteins produced by plasma cells. They bind to any foreign particles and invade them. Foreign pathogens are known as antigens.
Antigens invade the body and are capable of prompting a response from the immune system. This response may be combined with larger molecules or alone after binding to antibodies for a specific immune response. Thus, antigens stimulate the production of antibodies in the immune systems.
The Structure of Antibodies
Antibodies are “Y-shaped structures” that are composed of four polypeptide subunits. Every subunit has two identical heavy and light chains. Each heavy chain has an N-terminus, which forms an antigen-binding domain with a light chain.
There are two types of antigen-binding domains that form the arms of the “Y” shape. These are known as fragment antigen-binding (Fab) domains.
The heavy chains also have a C-terminus which forms a fragment crystallization (FC) domain that aids in the interaction with effector cells. The four polypeptide subunits are held together by noncovalent and disulfide bonds.
The heavy chains on antibodies have a variable region and three constant regions. Every antibody has two identical antigen-binding sites, but they differ in all antibodies.
Production and Mechanism of Antibody
When our immune systems initially encounter foreign organisms, macrophages interfere and capture these foreign particles to break them down and pass them to B cells.
When these antigens are presented, the B cells begin the production of a new antibody that will contain a unique paratope (a site where the antibody binds with antigen) that binds with a specific epitope (a site where antigen binds with antibody).
Every lymphocyte of the B cells creates an antibody against an epitope. After the encoding is completed by the B cells, it releases antibodies that will bind to specific pathogens and help expel them from our bodies.
Sometimes the use of a custom antibody is necessary to help recognize any unique molecules in the pathogen or a novel biomarker in the pathogens to remove them more effectively.
Antibodies have various functions in our bodies.
Antibodies play essential roles in the immune system, some of which are highlighted in detail below:
Activates The Immune System To Combat Bacterial Pathogens
Antibodies activate the immune systems by attaching to specific antigens to make it easier for the immune cells to destroy them. T lymphocytes attack all the antigens directly and help control the immune system’s response.
The T lymphocytes also release a chemical called cytokines, which also controls the whole immune response.
Neutralize Toxins and Infectivity
Antibodies are unusually found in the blood and mucosa, and here they can block any infective pathogens such as bacteria, viruses, fungi, and parasites.
They also neutralize foreign substances, such as toxins, or inactivate them. Neutralization by antibodies usually happens when they interfere with an alien organism’s attachment to host tissue.
Additionally, some antibodies have been shown to stop the infectivity process by binding to these foreign organisms and causing them to aggregate. Aggregation or agglutination by the antibodies will allow them to entrap the bacteria in mucous effectively.
However, the aggregation will more likely occur with polymeric IgM and IgA, but some neutralizing IgG antibodies may aggregate the poliovirus and lower their infectivity.
Antibodies help in the phagocytosis of any foreign organisms through a process called opsonization. Antibodies internalize and degrade pathogens by coating them through macrophages and neutrophils via FcRs is an essential function of antibodies and helps clear pathogens in vivo.
The binding of phagocyte Fc receptors by several antibody molecules with the same target signals a transduction pathway, resulting in the phagocytosis of the antigen-antibody complex. While inside the phagocyte, the pathogen is the main target of destructive processes, including enzymatic digestion, oxidative damage, and membrane disruption effects.
Lysis of Pathogens and Infected Cells
Antibodies initiate the complement system, resulting in the lysis of organisms or any infected cells. One important by-product of the complement process is C3b, a protein fragment that binds to the cell and other complexes. Many cell types have receptors for C3b, such as red blood cells or macrophages.
The binding of complexes by the C3b red blood cells receptors allows it to deliver to the spleen or liver, where other macrophages expel them without damaging the red blood cells.
Helps With Antibody-Dependent Cytotoxicity of Cells (ADCC)
Antibodies are shown to have anti-microbial activity directly or through their interactions with FcRs. ADCC happens when the antibody forms a bridge between the virus-infected cells of the host and the FcR-bearing effector cell, specifically natural killer cells. The result of this interaction is the death of target cells either by apoptosis or lysis.
Mucosal Immunity, Transcytosis, and Neonatal Immunity
Some antibodies have access across epithelial layers through a process called transcytosis. IgA is the immunoglobulin that goes through transcytosis and is usually found in a secretory form on the mucosal surfaces of gastrointestinal, respiratory, and urogenital tracts.
Antibodies’ functions in the body can sometimes be a double-edged sword. With one edge, they’ll protect the body from pathogens and microbes and, with the other, cause some very severe allergic reactions to less harmless proteins and other molecules found in food, medicine, and the environment. IgE is one of the most important mediators in the body for allergic reactions and hypersensitivity. When it binds to the multivalent antigens, there will be an activation of the mast cells which then release chemical mediators found in granules and mediate any inflammatory reactions.