Antibody

Antibody proteins are Y-shaped proteins that recognise and neutralise pathogens in the immune system. Antibodies play an important role in immune defence. Antibody structure consists of four polypeptide chains—two heavy and two light—and they form a distinctive structure with variable and constant regions. There are five main types of antibodies: IgM, IgG, IgA, IgD, and IgE, each having a specific role in the immune responses. Antibody functions include recognising and binding to specific antigens, marking them for destruction or neutralisation by the immune system.

The antigen, or distinctive molecule of the infection, is recognised by the antibody. These two structures can bind together precisely because each Y-shaped tip of an antibody has a paratope (like a lock) that is specific for one particular epitope (like a key) on an antigen.

Structure of the Antibody

Antibodies, also known as immunoglobulins (Ig), have a Y-shaped structure.

In detail, the antibody molecule structure is described below:

• Each arm of the Y consists of two types of protein chains—heavy chains and light chains.
• Variable regions are present on the tips of the Y-shaped arms. It recognises and binds to specific antigens.
• Constant region is present on the lower part of the Y-shaped arm, including the stem. It determines the class of the antibody (IgA, IgD, IgE, IgG, or IgM).
• Antigen binds at the tip of the Y-shaped antibody. The region where it binds is called the antigen-binding site.
• The light and heavy chain binds together through disulfide bonds. It helps in maintaining the structural integrity of antibodies.
• The stem of the Y is also known as the Fc region. It is involved in various immune responses, such as binding to cell receptors or activating complement proteins.

Types of Antibodies

An antibody (Ab) is a large, Y-shaped protein that the immune system utilises to recognise and destroy foreign substances, including harmful bacteria and viruses. They are spread throughout the body and have several functions.

There are 5 types of antibodies based on antibody structure and function that are given below:

1. IgG

IgG is a Y-shaped molecule with two antigen-binding sites.

• It is the primary antibody in the blood.
• It has a strong affinity for binding to bacteria and toxins.
• It is important to the biological defence system.
• IgG from the mother's body protects a baby since it is the only isotype that can cross the placenta.

2. IgM

IgM is pentameric structure with ten antigen-binding sites

• IgM is mostly found in the blood and is made up of five basic Y-shaped unit cells.
• After pathogen invasion, it is the first to be produced.
• It provides early response to infections.

3. IgA

IgA has a monomeric form in blood and a dimeric form with two Y-shaped molecules linked by a J chain in mucosal secretions.

• It also forms dimers, or molecules with two Ys, in secretions like intestinal fluid, nasal discharge, and saliva to thwart bacterial invasion from mucosal membranes.
• Additionally, found in breast milk, it shields a newborn's digestive system from viral and bacterial illness.

4. IgD

IgD is a monomeric molecule similar to IgG.

• It is found on the surface of B cells.
• IgD acts as a membrane receptor. 
• It aids in the stimulation of antibody synthesis.
• It also helps in the protection against respiratory tract infections.

5. IgE

IgE shows a monomeric structure.

• IgE was thought to have its origins in immune responses to parasites.
• It plays a role in allergies like pollinosis by attaching to mast cells.
• It triggers the release of histamines from mast cells and basophils.

Production and Mechanism of Action of Antibodies

B lymphocytes, often known as B cells, are specialised white blood cells that produce antibodies. A clone is a collection of identical cells that are created when a B cell divides and matures in response to an antigen binding to the B cell surface. Millions of antibodies are secreted into the lymphatic and circulation by mature B cells, also known as plasma cells.

Mechanism of Action of Antibodies

The immune system can identify foreign substances when they enter the body because the antigen's surface molecules are different from those found in the body. The immune system uses a number of methods, including one of the most crucial ones, antibody production, to get rid of the pathogen.

The mechanism of action of the antibodies is as follows:

• Antibodies recognise foreign substances or pathogens and bind to antigens.
• Antibodies neutralise the activity or function of pathogens by binding to them. It prevent pathogen from interacting with host cells.
• Antibodies increase phagocytosis by binding to pathogens. It marks them for destruction by phagocytic cells such as macrophages and neutrophils.
• Antibodies can destroy the pathogens through different processes, such as:
• Causes the aggregation of pathogens, that make it easier for phagocytes to engulf and eliminate pathogens,
• Cause soluble antigens to become insoluble. It forms immune complexes that are more easily removed by phagocytic cells. It cross-links multiple antigens that result in the formation of large complexes that are easily removed by immune cells
• Can stimulate NK cells to release cytotoxic substances. It results in the elimination of infected or abnormal cells.
• Antibodies play an important role in immunological memory. When the same antigen enters the body, the memory B cells quickly produce an antibody response. It provides a faster and more effective immune reaction.
• Antibodies show various modes of action to eliminate, neutralise, and regulate immune responses. It contributes to the body's defence against pathogens and foreign substances.

Functions of the Antibodies

The following are some of the functions of antibodies:

1. Antibodies recognise and bind to a specific antigen.
2. It blocks the toxins and harmful effects of pathogens by neutralising them.
3. It marks the pathogen for destruction by immune cells.
4. Antibodies trigger the complement system for pathogen destruction.
5. It stimulates natural killer cells to destroy infected cells.
6. It forms memory for rapid and robust response upon re-exposure to the same pathogen.
7. It regulates the various immune responses.

Applications of Antibodies in Medicines

In medicine, scientists have found many ways to use these antibodies to help people stay healthy:

1. Treating Diseases: Antibodies can be used as medicines to target and destroy specific harmful substances in the body, such as cancer cells or disease-causing bacteria.

2. Vaccines: Vaccines contain weakened or harmless versions of viruses or bacteria, which help our bodies produce antibodies to fight off those germs. This helps us build immunity against diseases like measles or the flu.

3. Diagnostic Tests: Antibodies can also be used in tests to detect if someone has a particular disease. For example, in COVID-19 tests, antibodies help identify if a person has been infected with the virus in the past.

4. Targeted Therapy: Some medicines use antibodies to specifically target certain cells in the body, like cancer cells, while leaving healthy cells unharmed. This helps reduce side effects and make treatments more effective.

Difference between Antigen and Antibody

The following points highlight the differences between antigens and antibodies:

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